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This paper considers the velocity control problem of nonlinear hydraulic servo systems subjected to unknown and time-varying external load disturbances. Hydraulically operated processes are usually represented by a hydraulic actuator-load system whose dynamic characteristics are complex and highly nonlinear, owing either to the flow-pressure relationships of the hydraulic system or to a load system motion itself. Furthermore, these characteristics are sometimes unknown owing to the uncertainty in external load disturbances to the processes. Therefore, the conventional approach to the controller design of these systems may not assure satisfactory control performance. To obtain better performance an adaptive model following control scheme was derived based upon Lyapunov's direct method . In order to deal with the uncertainties that are associated with the plant dynamics and the unknown disturbances, this method uses a small ultimate bound of the state error as an adaptation criterion. A series of simulation studies were performed to demonstrate the effectiveness of this controller. The results show that the proposed AMFC is fairly robust to unknown and timevarying external load disturbances, yielding improved performance characteristics when compared with a suboptimal PID controller with constant feedback gain.