A hammer tacker of the present invention includes a handle, a magazine, a striker, an actuator, a transmission mechanism and an abutting element. The handle is formed with a head at one end thereof and defines a receiving chamber therein. The magazine is disposed in the receiving chamber to receive nailing units. The striker is linear-slidably disposed in the head to strike the nailing units. The actuator is linear-slidably disposed on the head. The transmission mechanism connects the actuator with the striker for the actuator and the striker to move in opposite directions. The abutting element is pivoted on the actuator. And the actuator moves upward as the abutting element hits an object.