The influence of the relief valve, which is used as a safety valve, on the performances of a pump/inverter controlled hydraulic motor system is discussed. The mathematical models are derived, and then the optimization technique is used to identify the system parameters. A series of simulations and experiments are performed; the results show that a better simulation result can be obtained if the relief valve characteristics are considered in the system model, and adverse influence of the relief valve on the performance of the pump or inverter controlled hydraulic motor system can be avoided if a self-tuning adaptive controller is employed.