A method of variable structure control (VSC) using the frequency criteria of H∞ control theory for sliding surface design is proposed in windmill power systems. Using quasi-linearization transform technique to the windmill power systems, we develop a type of equation which is suitable for H∞ control full information problem. By solving the Riccati equations arising in H∞ control a stable sliding surface is decided. Then we design the control law which consists of a linear control law and a nonlinear control law to drive the state variable into the sliding surface and thereafter maintain within this subspace. The capability of the proposed controller to damp out the oscillations of the power and angular velocity, and the robustness with respect to the system parameter variations and model errors are evaluated in the simulation study.