A magazine with a positioning device for a nail gun includes a magazine that has a nail loading port on one end thereof, and a gliding part that has a protruding part and a locking part. The magazine has fixing parts, penetrating parts and a stopping part. The gliding device has a nail storing part and is inserted into the receiving chamber. The gliding device is fixed by a positioning device, so that the gliding device will not depart from the magazine due to the bounce motion while nailing, and the protruding part and the locking part of the gliding device are not easy to get abrasion. The objectives of providing the operator a safe and secured working environment, prolonging the lifetime of the gliding device and securing the gliding device in position will be attained.