This paper proposes a new approach for combining genetic algorithm and sliding mode control to design the power system stabilizers (PSS). The design of a PSS can be formulated as an optimal linear regulator control problem. However, implementing this technique requires the design of estimators. This increases the implementation and reduces the reliability of control system. These reasons, therefore, favor a control scheme that uses only some desired state variables, such as torque angle and speed. To deal with this problem, we use the optimal reduced models to reduce the power system model into two state variables system by each generator. We use the genetic algorithm to find the switching control signals and use sliding mode control to find control signal of the generator. The advantages of the proposed method are illustrated by numerical simulation of the multi-machine power systems.