Magnetorheological (MR) dampers are the semi-active control devices that use MR fluids to produce controllable dampers. In this paper, the design method of the cylindrical MR damper is established. Bingham model is used to describe the constitutive behavior of MR fluids subject to an applied magnetic field. The equation for the damping force is derived to provide the theoretical foundation for the design of the damper. Based on this equation the engineering expressions for the gap and effective length of the annular MR fluid between the piston and the outer cylinder are derived. The result shows that with the increase of the applied magnetic field strength, the damping force is increased. The dimensions of the gap and length can be calculated when the required damping force, the velocity of the piston, and the desired control damping force ratio are specified.