The optimal performance of power system stabilizers (PSSs), in multimachine power systems, depends highly on their parameters, location and number. This paper presents an approach of simultaneous optimization of PSS parameters, location and number in multimachine power systems based eigenvalue multiobjective function and solving using genetic algorithms (GAs). This approach permits to avoid using the conventional PSS location methods which may not guarantee the effectiveness of system damping, especially for inter-area modes where a large number of generators may be involved in the oscillations. To validate the proposed approach performance, the New England/New York multimachine system (16-generator and 68-bus) has been employed. Eigenvalues analysis and nonlinear system simulation results show the effectiveness of the proposed approach to damp out the electromechanical oscillation and enhance the global system stability.