MR Fluid, one of functional fluids, is developed for the application to automobile products. MR CDC damper using MR fluid has following principles. When ar electric current is applied to the solenoid, apparent viscosity of MR fluid passing through the annular gap which acts as magnetic circuits varies directly as the intensity of the current. These devices have a simple structure and excellent lime response characteristics, emerging as the alternatives of the conventional semi-active suspension systems. In this study, a design procedure of the magnetic circuit through the solenoid fore and the flux ring functioning as a magnetic path is investigated so as to optimize the design and performance of MR CDC dampers for the vehicles. In addition, an operating point on the B-H curve, the magnetization according to the variation in the annular gap, the pole piece width and the density of MR fluid are studied to design the optimal piston head within the restrained dimension range.