The power system stabilizer (PSS) is considered to be an effective facility for improving the dynamic stability. In this paper, a nonlinear optimization based on Goh's formulation is used to determine the PSS parameters. The real part of dominant eigenvalue of the system is minimized by tuning the PSS parameters through the formulation incorporated with the genetic algorithms (GA). Numerical results for a one-machine system show that the proposed method can effectively improve the power system dynamic stability.