This paper presents an optimal design of Fuzzy Power System Stabilizer (FPSS) using Harmony Search Algorithm (HSA) in multi-machine power system. The PID controller is one of the appropriate techniques to damp low frequency oscillation in nonlinear systems. The proposed controller is based on their rule base and membership function information. In conventional techniques the information is obtained by trial and error which needs too much time. So, finding the optimum value of rule base or membership function is really important. Accordingly, HAS is proposed in this paper to find the optimum value of rule base by considering the power system constrains and factors. This newly developed control strategy mixed the advantage of HSA and Fuzzy controller with simple structure while is easy to implement. Hence, the proposed controller is tested over the New England 10-unit 39-bus standard power system and 9 buses IEEE power system, under various system configurations and loading conditions, to illustrate the performance of the proposed method. The effectiveness of proposed controller is compared with other techniques. Eigenvalue analysis and nonlinear simulation results show the effectiveness of the proposed controller