An investigation of the performance of pilot operated hydraulic relief valves has been carried out. The valve behavior is described by means of a nonlinear mathematical model, considering the nonlinear variation of the restriction areas, the nonlinear flow-pressure relations and the flow forces. The theoretical treatment is carried out by treating the developed mathematical model using a digital simulation program. The model is validated in the steady state by comparing simulation and experimental results. The transient behavior of the valve is investigated theoretically. The effect of the dimensions of the damping orifices and volume of oil filling the inlet line is predicted. The simulation program is used to predict the optimum dimensions of the damping orifices, which introduced an observable improvement in the transient response.