Design optimization of Magneto-rheological (MR) dampers is important to meet engineering applications and obtain MR dampers with better performance compared with that designed by conventional design methods. Optimal design of MR dampers in vehicle based on Finite Element (FE) is proposed. A mathematical model to describe optimization problem is constructed. In the optimization, the special cylindrical volume filled with active fluid is considered as the objective function, and the controllable range of damping force is constrained to satisfy the requirements of semi-active control system in vehicle. An optimization procedure is constructed via a commercial finite element method parametric design language. Optimization design of the MR damper with valve mode in vehicle is performed by using the optimization tool developed. The results show that the optimal MR damper has minimum volume, reasonable magnetic flux density distribution, effective energy consumption and the controllable damping force to satisfy the requirement of semi-active control system in vehicle. The results demonstrate the effectiveness and feasibility of the proposed optimization approach.