Hydraulic engine mounts are replacing conventional rubber mounts in order to provide better ride quality and to reduce noise. In this paper, a real-time semi-active control method of hydraulic mount damping is presented. Although it is desirable to use the state of fluid motion as the feedback signal, it is difficult to measure this mortion directly. As an alternative, a simulator which estimates fluid motion from the measurable vehicle vibration is developed. However, real-time control is limited by the speed of computation; thus it is of interest to find optimal numerical integration algorithms. Of the Euler, Adams-Bashforth, Runge-Kutta and central difference methods, the latter method is the most feasible for this application because steady-state error becomes negligible even with large sampling periods. It is demonstrated experimentally that this simulator is effective for vibration control.