A door closer capable of adjusting door closing speed, includes a pneumatic cylinder on the door frame, a hydraulic cylinder on the door, and a lever. One end of the lever is movably connected to the pneumatic cylinder, and the other end is connected to the hydraulic cylinder. The pneumatic cylinder includes a sliding rail, and a sliding member cooperated with the sliding rail. A hermetic chamber is formed by the sliding member and the sliding rail. An adjusting valve, for adjusting the air exhaust of the hermetic chamber, is provided on a wall of the hermetic chamber.