This paper describes a new generator's PSS (power system stabilizer) design method. A multi input PSS which is designed by considering several power flow conditions is utilized to achieve a better robustness over a wide range of operating conditions and oscillation frequency, instead of a conventional single input PSS. The parameters of the optimized PSS are obtained by minimizing a performance index based on eigenvalues that shows steady state stability of the power system The method utilizes eigenvalue sensitivities associated with PSS parameters for each pattern of multiple power flow conditions. Simulation results show that the proposed PSS provides a better performance than the conventional PSS.