A hammer tacker includes a shell and a striking mechanism. The shell is composed of two half-shells which are made of a mixture of 30% to 70% of fiberglass and 70% to 30% of nylon by weight by injection molding. The two half-shells enclose an opening at an end of the shell. Each half-shell has a positioning rib portion facing to the other half-shell. The striking mechanism is arranged in the shell and includes a magazine, a striker, an actuator, and a transmission mechanism. The magazine is positioned by the positioning rib portions. The striker is located at the opening and is movable for striking nails out. The actuator is also movable at the opening. The transmission mechanism connects the striker and the actuator together to make them move oppositely.