A negative damping stabilizer (NDS) with optimal parameters was designed to suppress the self-excited oscillations occurring in power systems. The suggested NDS controller was applied to a single machine connected to an infinite power system through a series compensated tie line. Genetic algorithms (GAs) were applied to determine the NDS parameters on a basis of enhancing the damping over the modes most susceptible to instability. The system performance was investigated under both small perturbations and large scale disturbances. The results obtained confirmed the effectiveness of the suggested controller.