A sliding mode controller has been used to determine the mechanism of uncertain and nonlinear control systems. A sliding mode control (SMC) system is generally characterized by its ability to maintain the direction of the state trajectory along the switching surface and the state trajectory converges to the origin. In this study, the SMC technique is employed to control the positioning of an oil hydraulic cylinder. Moreover, one difficulty in applying the SMC is the need to obtain the values of all state vectors. To solve this problem, we applied a robust reduced-order VSS observer based on the sliding mode control theory. The experiments of the SMC are carried out under several conditions, and the obtained results are compared with those of the conventional PID controller. The SMC is found to show the good tracking performance and robustness of the response compared to the conventional PID controller under several conditions of external loads.