Detailed knowledge of actuator properties is a prerequisite for advanced manipulator design and control. This paper deals with the experimental identification and modelling of the nonlinear dynamics ofa high performance hydraulic actuator. Such actuators are of interest for applications which require both high power and high bandwidth. An analytical model of the system is formulated, and a software simulator implementing the force-controlled actuator model including all the nonlinear elements is shown to predict the real system's behavior quite well. The actuator properties and performance are also discussed.