diff --git a/evaluator/evaluator.go b/evaluator/evaluator.go
index 632ad2b..241c241 100644
--- a/evaluator/evaluator.go
+++ b/evaluator/evaluator.go
@@ -56,6 +56,8 @@ func eval_prefix_expression(operator string, right object.Object) object.Object
 	switch operator {
 	case "!":
 		return eval_bang_operator_expression(right)
+	case "-":
+		return eval_minus_prefix_operator_expression(right)
 
 	default:
 		return NULL
@@ -77,3 +79,11 @@ func eval_bang_operator_expression(right object.Object) object.Object {
 		return FALSE
 	}
 }
+
+func eval_minus_prefix_operator_expression(right object.Object) object.Object {
+	if right.Type() != object.INTEGER_OBJECT{
+		return NULL
+	}
+	value := right.(*object.Integer).Value
+	return &object.Integer{Value: -value}
+}
diff --git a/evaluator/evaluator_test.go b/evaluator/evaluator_test.go
index cb6e505..63055ff 100644
--- a/evaluator/evaluator_test.go
+++ b/evaluator/evaluator_test.go
@@ -14,6 +14,9 @@ func TestEvalIntegerExpression(l_test *testing.T) {
 	}{
 		{"5", 5},
 		{"10", 10},
+		{"-10", -10},
+		{"-5", -5},
+
 	}
 
 	for _, tt := range tests {