This research paper presents a technique based on the non- linear optimization method for parameters of the power system stabilizers in multimachine power system. For the optimization or tuning of the parameters of PSS, the objective function is the maximization of the damping ratio of the critical modes of electro mechanical rotor oscillations. The objective of the coordinated parameter tuning is to globally optimize the overall system damping performance by maximize the damping of all both local and inter area modes of oscillations. The time domain simulation results of the 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.