This paper presents a new method based on the optimization technique for the interconnected simultaneous tuning of the parameters of all the power system stabilizers in multimachine power system. For tuning the parameters of PSS, non-linear optimization technique is used. In this non-linear optimization, the objective function is the maximization of damping ratio of the critical modes of rotor oscillations. The time domain simulation results of multi machine power system validate the effectiveness of the proposed approach. In this paper, the 10 machine 39bus New England system is used as the test system.