This paper deals with the rotational velocity control of a water hydraulic servomotor system. Water hydraulic servo system is an attractive technology for safety environment. Considering the parameter uncertainties, e.g. load fluctuation, friction parameters, parameter estimation error etc., the Lyapunov-based recursive method is applied to design a robust rotational velocity controller. The proposed controller ensures that the set to which state error approaches can be made arbitrary small by choosing the control gain. This is called the uniformly ultimate bounded stability. The effectiveness of the proposed controller is examined numerically.