diff --git a/web/src/components/CompetitionPage.test.tsx b/web/src/components/CompetitionPage.test.tsx
new file mode 100644
index 00000000..6b06139d
--- /dev/null
+++ b/web/src/components/CompetitionPage.test.tsx
@@ -0,0 +1,329 @@
+import { describe, it, expect } from 'vitest'
+
+/**
+ * PR #678 測試: 修復 CompetitionPage 中 NaN 和缺失數據的顯示問題
+ *
+ * 問題：當 total_pnl_pct 為 null/undefined/NaN 時，會顯示 "NaN%" 或 "0.00%"
+ * 修復：檢查數據有效性，顯示 "—" 表示缺失數據
+ */
+
+describe('CompetitionPage - Data Validation Logic (PR #678)', () => {
+  /**
+   * 測試數據有效性檢查邏輯
+   * 這是 PR #678 引入的核心邏輯
+   */
+  describe('hasValidData check', () => {
+    it('should return true for valid numbers', () => {
+      const trader1 = { total_pnl_pct: 10.5 }
+      const trader2 = { total_pnl_pct: -5.2 }
+
+      const hasValidData =
+        trader1.total_pnl_pct != null &&
+        trader2.total_pnl_pct != null &&
+        !isNaN(trader1.total_pnl_pct) &&
+        !isNaN(trader2.total_pnl_pct)
+
+      expect(hasValidData).toBe(true)
+    })
+
+    it('should return false when trader1 has null value', () => {
+      const trader1 = { total_pnl_pct: null }
+      const trader2 = { total_pnl_pct: 10.5 }
+
+      const hasValidData =
+        trader1.total_pnl_pct != null &&
+        trader2.total_pnl_pct != null &&
+        !isNaN(trader1.total_pnl_pct!) &&
+        !isNaN(trader2.total_pnl_pct)
+
+      expect(hasValidData).toBe(false)
+    })
+
+    it('should return false when trader2 has undefined value', () => {
+      const trader1 = { total_pnl_pct: 10.5 }
+      const trader2 = { total_pnl_pct: undefined }
+
+      const hasValidData =
+        trader1.total_pnl_pct != null &&
+        trader2.total_pnl_pct != null &&
+        !isNaN(trader1.total_pnl_pct) &&
+        !isNaN(trader2.total_pnl_pct!)
+
+      expect(hasValidData).toBe(false)
+    })
+
+    it('should return false when trader1 has NaN value', () => {
+      const trader1 = { total_pnl_pct: NaN }
+      const trader2 = { total_pnl_pct: 10.5 }
+
+      const hasValidData =
+        trader1.total_pnl_pct != null &&
+        trader2.total_pnl_pct != null &&
+        !isNaN(trader1.total_pnl_pct) &&
+        !isNaN(trader2.total_pnl_pct)
+
+      expect(hasValidData).toBe(false)
+    })
+
+    it('should return false when both traders have invalid data', () => {
+      const trader1 = { total_pnl_pct: null }
+      const trader2 = { total_pnl_pct: NaN }
+
+      const hasValidData =
+        trader1.total_pnl_pct != null &&
+        trader2.total_pnl_pct != null &&
+        !isNaN(trader1.total_pnl_pct!) &&
+        !isNaN(trader2.total_pnl_pct)
+
+      expect(hasValidData).toBe(false)
+    })
+
+    it('should handle zero as valid data', () => {
+      const trader1 = { total_pnl_pct: 0 }
+      const trader2 = { total_pnl_pct: 10.5 }
+
+      const hasValidData =
+        trader1.total_pnl_pct != null &&
+        trader2.total_pnl_pct != null &&
+        !isNaN(trader1.total_pnl_pct) &&
+        !isNaN(trader2.total_pnl_pct)
+
+      expect(hasValidData).toBe(true)
+    })
+
+    it('should handle negative numbers as valid data', () => {
+      const trader1 = { total_pnl_pct: -15.5 }
+      const trader2 = { total_pnl_pct: -8.2 }
+
+      const hasValidData =
+        trader1.total_pnl_pct != null &&
+        trader2.total_pnl_pct != null &&
+        !isNaN(trader1.total_pnl_pct) &&
+        !isNaN(trader2.total_pnl_pct)
+
+      expect(hasValidData).toBe(true)
+    })
+  })
+
+  /**
+   * 測試 gap 計算邏輯
+   * gap 應該只在數據有效時計算
+   */
+  describe('gap calculation', () => {
+    it('should calculate gap correctly for valid data', () => {
+      const trader1 = { total_pnl_pct: 15.5 }
+      const trader2 = { total_pnl_pct: 10.2 }
+
+      const hasValidData =
+        trader1.total_pnl_pct != null &&
+        trader2.total_pnl_pct != null &&
+        !isNaN(trader1.total_pnl_pct) &&
+        !isNaN(trader2.total_pnl_pct)
+
+      const gap = hasValidData
+        ? trader1.total_pnl_pct - trader2.total_pnl_pct
+        : NaN
+
+      expect(gap).toBeCloseTo(5.3, 1) // Allow floating point precision
+      expect(isNaN(gap)).toBe(false)
+    })
+
+    it('should return NaN for invalid data', () => {
+      const trader1 = { total_pnl_pct: null }
+      const trader2 = { total_pnl_pct: 10.2 }
+
+      const hasValidData =
+        trader1.total_pnl_pct != null &&
+        trader2.total_pnl_pct != null &&
+        !isNaN(trader1.total_pnl_pct!) &&
+        !isNaN(trader2.total_pnl_pct)
+
+      const gap = hasValidData
+        ? trader1.total_pnl_pct! - trader2.total_pnl_pct
+        : NaN
+
+      expect(isNaN(gap)).toBe(true)
+    })
+
+    it('should handle negative gap correctly', () => {
+      const trader1 = { total_pnl_pct: 5.0 }
+      const trader2 = { total_pnl_pct: 12.0 }
+
+      const hasValidData =
+        trader1.total_pnl_pct != null &&
+        trader2.total_pnl_pct != null &&
+        !isNaN(trader1.total_pnl_pct) &&
+        !isNaN(trader2.total_pnl_pct)
+
+      const gap = hasValidData
+        ? trader1.total_pnl_pct - trader2.total_pnl_pct
+        : NaN
+
+      expect(gap).toBe(-7.0)
+    })
+  })
+
+  /**
+   * 測試顯示邏輯
+   * 修復後應顯示「—」而非「NaN%」或「0.00%」
+   */
+  describe('display formatting', () => {
+    it('should format valid positive percentage correctly', () => {
+      const total_pnl_pct = 15.567
+
+      const display =
+        total_pnl_pct != null && !isNaN(total_pnl_pct)
+          ? `${total_pnl_pct >= 0 ? '+' : ''}${total_pnl_pct.toFixed(2)}%`
+          : '—'
+
+      expect(display).toBe('+15.57%')
+    })
+
+    it('should format valid negative percentage correctly', () => {
+      const total_pnl_pct = -8.234
+
+      const display =
+        total_pnl_pct != null && !isNaN(total_pnl_pct)
+          ? `${total_pnl_pct >= 0 ? '+' : ''}${total_pnl_pct.toFixed(2)}%`
+          : '—'
+
+      expect(display).toBe('-8.23%')
+    })
+
+    it('should display "—" for null value', () => {
+      const total_pnl_pct = null
+
+      const display =
+        total_pnl_pct != null && !isNaN(total_pnl_pct)
+          ? `${total_pnl_pct >= 0 ? '+' : ''}${total_pnl_pct.toFixed(2)}%`
+          : '—'
+
+      expect(display).toBe('—')
+    })
+
+    it('should display "—" for undefined value', () => {
+      const total_pnl_pct = undefined
+
+      const display =
+        total_pnl_pct != null && !isNaN(total_pnl_pct)
+          ? `${total_pnl_pct >= 0 ? '+' : ''}${total_pnl_pct.toFixed(2)}%`
+          : '—'
+
+      expect(display).toBe('—')
+    })
+
+    it('should display "—" for NaN value', () => {
+      const total_pnl_pct = NaN
+
+      const display =
+        total_pnl_pct != null && !isNaN(total_pnl_pct)
+          ? `${total_pnl_pct >= 0 ? '+' : ''}${total_pnl_pct.toFixed(2)}%`
+          : '—'
+
+      expect(display).toBe('—')
+    })
+
+    it('should format zero correctly', () => {
+      const total_pnl_pct = 0
+
+      const display =
+        total_pnl_pct != null && !isNaN(total_pnl_pct)
+          ? `${total_pnl_pct >= 0 ? '+' : ''}${total_pnl_pct.toFixed(2)}%`
+          : '—'
+
+      expect(display).toBe('+0.00%')
+    })
+  })
+
+  /**
+   * 測試領先/落後訊息顯示邏輯
+   * 只有在數據有效時才顯示 "領先" 或 "落後" 訊息
+   */
+  describe('leading/trailing message display', () => {
+    it('should show leading message when winning with positive gap', () => {
+      const isWinning = true
+      const gap = 5.2
+      const hasValidData = true
+
+      const shouldShowLeading = hasValidData && isWinning && gap > 0
+
+      expect(shouldShowLeading).toBe(true)
+    })
+
+    it('should not show leading message when data is invalid', () => {
+      const isWinning = true
+      const gap = NaN
+      const hasValidData = false
+
+      const shouldShowLeading = hasValidData && isWinning && gap > 0
+
+      expect(shouldShowLeading).toBe(false)
+    })
+
+    it('should show trailing message when losing with negative gap', () => {
+      const isWinning = false
+      const gap = -3.5
+      const hasValidData = true
+
+      const shouldShowTrailing = hasValidData && !isWinning && gap < 0
+
+      expect(shouldShowTrailing).toBe(true)
+    })
+
+    it('should not show trailing message when data is invalid', () => {
+      const isWinning = false
+      const gap = NaN
+      const hasValidData = false
+
+      const shouldShowTrailing = hasValidData && !isWinning && gap < 0
+
+      expect(shouldShowTrailing).toBe(false)
+    })
+
+    it('should show fallback "—" when data is invalid', () => {
+      const hasValidData = false
+
+      const shouldShowFallback = !hasValidData
+
+      expect(shouldShowFallback).toBe(true)
+    })
+  })
+
+  /**
+   * 測試邊界情況
+   */
+  describe('edge cases', () => {
+    it('should handle very small positive numbers', () => {
+      const total_pnl_pct = 0.001
+
+      const hasValidData = total_pnl_pct != null && !isNaN(total_pnl_pct)
+
+      expect(hasValidData).toBe(true)
+    })
+
+    it('should handle very large numbers', () => {
+      const total_pnl_pct = 9999.99
+
+      const hasValidData = total_pnl_pct != null && !isNaN(total_pnl_pct)
+
+      expect(hasValidData).toBe(true)
+    })
+
+    it('should handle Infinity as invalid (produces NaN in calculations)', () => {
+      const total_pnl_pct = Infinity
+
+      // Infinity 本身不是 NaN，但在減法運算中可能導致問題
+      const hasValidData = total_pnl_pct != null && isFinite(total_pnl_pct)
+
+      expect(hasValidData).toBe(false)
+    })
+
+    it('should handle -Infinity as invalid', () => {
+      const total_pnl_pct = -Infinity
+
+      const hasValidData = total_pnl_pct != null && isFinite(total_pnl_pct)
+
+      expect(hasValidData).toBe(false)
+    })
+  })
+})
diff --git a/web/src/components/CompetitionPage.tsx b/web/src/components/CompetitionPage.tsx
index 2c1effd2..e74d119b 100644
--- a/web/src/components/CompetitionPage.tsx
+++ b/web/src/components/CompetitionPage.tsx
@@ -392,7 +392,17 @@ export function CompetitionPage() {
             {sortedTraders.map((trader, index) => {
               const isWinning = index === 0
               const opponent = sortedTraders[1 - index]
-              const gap = trader.total_pnl_pct - opponent.total_pnl_pct
+
+              // Check if both values are valid numbers
+              const hasValidData =
+                trader.total_pnl_pct != null &&
+                opponent.total_pnl_pct != null &&
+                !isNaN(trader.total_pnl_pct) &&
+                !isNaN(opponent.total_pnl_pct)
+
+              const gap = hasValidData
+                ? trader.total_pnl_pct - opponent.total_pnl_pct
+                : NaN
 
               return (
                 <div
@@ -429,10 +439,12 @@ export function CompetitionPage() {
                           (trader.total_pnl ?? 0) >= 0 ? '#0ECB81' : '#F6465D',
                       }}
                     >
-                      {(trader.total_pnl ?? 0) >= 0 ? '+' : ''}
-                      {trader.total_pnl_pct?.toFixed(2) || '0.00'}%
+                      {trader.total_pnl_pct != null &&
+                      !isNaN(trader.total_pnl_pct)
+                        ? `${trader.total_pnl_pct >= 0 ? '+' : ''}${trader.total_pnl_pct.toFixed(2)}%`
+                        : '—'}
                     </div>
-                    {isWinning && gap > 0 && (
+                    {hasValidData && isWinning && gap > 0 && (
                       <div
                         className="text-xs font-semibold"
                         style={{ color: '#0ECB81' }}
@@ -440,7 +452,7 @@ export function CompetitionPage() {
                         {t('leadingBy', language, { gap: gap.toFixed(2) })}
                       </div>
                     )}
-                    {!isWinning && gap < 0 && (
+                    {hasValidData && !isWinning && gap < 0 && (
                       <div
                         className="text-xs font-semibold"
                         style={{ color: '#F6465D' }}
@@ -450,6 +462,14 @@ export function CompetitionPage() {
                         })}
                       </div>
                     )}
+                    {!hasValidData && (
+                      <div
+                        className="text-xs font-semibold"
+                        style={{ color: '#848E9C' }}
+                      >
+                        —
+                      </div>
+                    )}
                   </div>
                 </div>
               )