Abstract: A shock absorber has an upper cylinder, a piston valve mounted on a lower end of the upper cylinder, a free piston slidably mounted in the upper cylinder and dividing the upper cylinder into an air chamber and an upper oil chamber, a lower cylinder slidably mounted around the lower end of the upper cylinder and has a lower oil chamber. Since the free piston and the air chamber are disposed in the upper cylinder, the shock absorber has a compact structure. Furthermore, when the upper and lower cylinders slide relatively, the free piston slides accordingly to regulate volume of the upper oil chamber and to provide a damping effect. The hydraulic oil in the shock absorber does not contact air and therefore no air permeates into the hydraulic oil.