•The rotational speed control system of the water-driven spindle was designed in the present paper.•The designed feedback control system can reduce the change in the spindle speed due to the cutting forces.•The disturbance observer was added into the feedback control system.•The influence of cutting force on spindle speed can be minimized the designed control system with disturbance observer. The water driven spindle has been developed for use in an ultra-precision machine tool for producing precision parts. In order to achieve high machining accuracy and generate high-quality surfaces, precise rotational accuracy and a constant machine tool spindle speed are necessary in ultra-precision machine tools. However, the rotational speed of the spindle inevitably changes due to the influence of the cutting forces during machining process. In order to deal this problem, this paper describes the development of a rotational speed control system. In particular, the control system is designed such that the influence of the cutting forces is effectively reduced. In this paper, mathematical models of the water-driven spindle and the flow control valve are introduced to develop a feedback control system. The effectiveness of the developed feedback control system is then verified through simulation and experimental tests. In addition, the disturbance observer is added to the designed feedback control system in order to minimize the influence of the cutting forces. Turning tests are then carried out in order to verify the effectiveness of the disturbance observer. As a results, the developed feedback control system with the disturbance observer is verified to successfully reduce the change in the rotational speed.