A stapler has a supporting base, a magazine assembly, a trigger assembly and a leg-flatting device. The trigger assembly has a trigger lever and a pushing element. The pushing element is mounted pivotally on the trigger lever with a pivot and has two pushing arms extending toward the supporting base. The leg-flatting device is mounted on the supporting base and has a sliding base, a moving base and an anvil element. The sliding base is slidably mounted on the supporting base and has a pushed segment corresponding to and selectively pushed by the pushing arms. The moving base is selectively blocked by the sliding base to keep the moving base from moving downwardly before the sliding base sliding relative to the supporting base and has an elongated hole. The anvil element is mounted in the elongated hole in the moving base.