A crossbow includes a barrel with a flight groove, and a bow. A magazine is mounted to the body and located above the flight groove. A passage is defined through the magazine for accommodating multiple arrows therein. A slot is formed between the magazine and the flight groove. A string of the bow movably extends through the slot. An arm is pivotably located in the passage and includes a hook end which normally presses on a top-most arrow of the arrows in the magazine. When no arrow is in the magazine, the hook end is inserted into the slot and the user cannot successfully cock the string because the string is stopped by the hook end, so that the user is acknowledged that there is no arrow in the magazine.