Abstract: A staple positioning device in a magazine assembly of stapling gun includes a positioning plate movably received in the magazine and having two inclined slots. A driving plate has two bosses which are movably engaged with the two inclined slots and an end of the driving plate is connected to a pushing member on an outside of the magazine assembly. A shuttle is movably connected to the magazine assembly and a first end member of the shuttle moves the pushing member when moving the shuttle. The vertical movement of the driving plate makes the positioning plate to move in a horizontal direction so as to urge staples with different sizes in the magazine assembly.