This paper presents application of a fractional order controller for a semi-active suspension system integrated with magnetorheological fluid damper. A quarter-car model is used in this research and only vertical motion of the vehicle is considered. In order to control the nonlinear behavior of the magnetorheological fluid damper (MRD) to achieve the optimal suspension performance, two controllers are designed for the whole system, namely a system controller and a damper controller. A fractional order proportional integral derivative (PID) controller is used as the system controller and a continuous state controller is employed as the damper controller. The Nelder-Mead optimization is adapted to tune the controller parameters according to the desire performance criteria. Simulation results clearly shows effectiveness of the proposed controllers as compared to a passive system and a traditional PID controlled MRD suspension system.