A shock absorber includes a housing, a cylinder, a piston rod, a piston and a chamber. The housing has an inner space containing a hydraulic fluid. The cylinder has an upper room filled with a magnetorhelogical fluid and a lower room filled with the hydraulic fluid. The piston rod is movably inserted in the upper room of the cylinder. The piston is movably inserted in the lower room of the cylinder, secured to the piston rod and structured to form a magnetic field generator. The chamber is mechanically connected to the piston rod. The piston rod further includes at least first orifice for providing a flowing path for the magnetorheological fluid and the piston further includes at least a second orifice for providing a flowing path for the hydraulic fluid in the lower room of cylinder. The shock absorber utilizes the hydraulic fluid and the magnetorheological fluid, which can, in turns, can respond sensitively to the impulse applied thereto by using the hydraulic fluid having relatively low viscosity and, at the same time, can modulate the damping force by using the magnetorheological fluid having changeable viscosity.