diff --git a/src/main/java/de/tims/tictactoe/ai/AIHard.java b/src/main/java/de/tims/tictactoe/ai/AIHard.java
index 99aee69..9768677 100644
--- a/src/main/java/de/tims/tictactoe/ai/AIHard.java
+++ b/src/main/java/de/tims/tictactoe/ai/AIHard.java
@@ -21,16 +21,33 @@ public class AIHard implements TicTacToeAI {
 	public void calculateNextMove() {
 		int charsOfOpponentInRow = 0;
 		int charsOfOpponentInCol = 0;
+		int charsOfOpponentInDiag = 0;
 		char[][] board = gl.getBoard();
 		
 		for (int i = 0; i < BOARD_SIZE; i++) {
 			charsOfOpponentInRow = countCharsInRow(i, PLAYER_CHAR);
 			charsOfOpponentInCol = countCharsInCol(i, PLAYER_CHAR);
-			if (charsOfOpponentInRow == BOARD_SIZE - 1 || charsOfOpponentInCol == BOARD_SIZE - 1) {
+			if (i < 2) {
+				charsOfOpponentInDiag = countCharsInDiag(i, PLAYER_CHAR);
+			}
+			
+			if (charsOfOpponentInRow == BOARD_SIZE - 1 || charsOfOpponentInCol == BOARD_SIZE - 1 || charsOfOpponentInDiag == BOARD_SIZE - 1) {
 				for (int j = 0; j < BOARD_SIZE; j++) {
-					if (board[(charsOfOpponentInRow == BOARD_SIZE - 1) ? i : j][(charsOfOpponentInRow == BOARD_SIZE - 1) ? j : i] == EMPTY_CHAR) {
-						gl.setField((charsOfOpponentInRow == BOARD_SIZE - 1) ? i : j, (charsOfOpponentInRow == BOARD_SIZE - 1) ? j : i, AI_CHAR);
-						return;
+					if (charsOfOpponentInRow == BOARD_SIZE - 1) {
+						if (board[i][j] == EMPTY_CHAR) {
+							gl.setField(i, j, AI_CHAR);
+							return;
+						}
+					} else if (charsOfOpponentInCol == BOARD_SIZE - 1) {
+						if (board[j][i] == EMPTY_CHAR) {
+							gl.setField(j, i, AI_CHAR);
+							return;
+						}
+					} else if (charsOfOpponentInDiag == BOARD_SIZE - 1) {
+						if (board[j][(i == 0) ? j : BOARD_SIZE - 1 - j] == EMPTY_CHAR) {
+							gl.setField(j, (i == 0) ? j : BOARD_SIZE - 1 - j, AI_CHAR);
+							return;
+						}
 					}
 				}
 			}
diff --git a/src/test/java/de/tims/tictactoe/ai/AIHardTest.java b/src/test/java/de/tims/tictactoe/ai/AIHardTest.java
index ecede92..dbadc8b 100644
--- a/src/test/java/de/tims/tictactoe/ai/AIHardTest.java
+++ b/src/test/java/de/tims/tictactoe/ai/AIHardTest.java
@@ -87,6 +87,17 @@ class AIHardTest {
 		
 		verify(gl, times(1)).setField(2, 1, realChar);
 	}
+
+	@Test
+	void preventOpponentsWinInDiag() {
+		char realChar = 'o';
+		doReturn(new char[][] { {'x', '-', 'o'}, {'-', 'x', '-'}, {'-', '-', '-'} }).when(gl).getBoard();
+		
+		TicTacToeAI ai = new AIHard(gl);
+		ai.calculateNextMove();
+		
+		verify(gl, times(1)).setField(2, 2, realChar);
+	}
 	
 	@ParameterizedTest
 	@MethodSource("testCasesForCountCharsInRow")