Abstract: In order to obtain a high precision control performance for nonlinear hydraulic servo system in the presence of large frictional torque, a novel control scheme based on the slidingmode variable structure controller combined with friction state observer was presented. The slidingmodel variable structure control law and the slidingmodel friction state observer were detailed. The LuGre dynamic friction model and the sliding mode friction observer were adopted for friction compensation. The experimental results show that high precision control can be accomplished in the presence of large friction torque and very low velocities.