In order to extract the maximal power from the wind under the rated wind speed, a sliding mode controller is designed in this paper for a variable speed wind turbine (VSWT), the proposed control algorithm possesses the properties to overcome the nonlinearity, uncertainty and external disturbance within VSWT, it drives the turbine rotor to rotate toward desired tracking speed to maximally harvest the wind power under the rated wind speed. The results of simulation studies show that the effectiveness and the advantages of the designed controller.