Abstract: Because the characteristic equation of fractional order system is a pseudo-polynomial function with exponential-type fractional orders of complex variable, the control of integer orders cannot be directly applied to it. A robust H∞ controller is therefore designed for fractional order system to which the filter approximation proposed is applied with a weighting function selected properly to meet the requirements for designing the H∞ controller. Simulation results indicate that the performance of the system thus controlled is significantly improved with dynamic behavior satisfactorily obtained. The effectiveness of the method proposed is verified through tests.