With the existence condition of a sliding regime and the occurrence of hitting, a new approach for determining the switching gains of an optimal variable structure stabilizer with parameter uncertainties is presented. Based on the bounds of uncertainties, the proposed algorithm is a systematic procedure. Simulation results show that with the proposed stabilizer, the dynamic performance of a synchronous generator system can greatly be improved and the dynamic response is better than the variable structure stabilizer with trial and error switching gains and the proportional-integral stabilizer. Furthermore, if the variations of the parameters are within the indicated range, then with the proposed switching gains, the system can always obtain a good performance.