This paper describes a theoretical investigation is the design and development of a variable damper which is used in semi active suspension systems. The control valve is a variable damper generally comprises a servo valve and a valve driving device. The description of the possible driving devices eg. solenoid, DC motor, torque motor and stepping motor are given. Also, the calculation of the diameter of the main orifice in the control valve is given. A variable damper is fitted in a theoretical model of the semi-active suspension system and the calculated results are obtained. Under all conditions of load and road surface the modified variable damper can be adjusted. This increase ride comfort considerably without loss of vehicle control.