A variable structure controller with time-varying sliding surface is proposed for robot manipulators. The proposed time-varing sliding surface ensures the existence of a sliding mode from an initial state, whereas the conventional sliding surface cannot achieve robust performance against parameter variations and disturbances before the sliding mode occurs. Therefore, error transients can be prescribed in advance for all time. Furthermore, it is shown that the overall system is globally exponentially stable. The eficiency of the proposed method for the trajectory tracking is demonstrated by an example.