We developed an on–line experimental system for conducting hybrid experiments in real time. It combines a computer, which conducts vibration simulation and generates a control signal, and a hydraulic actuator, which conducts a vibration experiment driven by the control signal. This system compensates for actuator delay and thus enables experiments to be carried out in real time. We evaluated the stability of the experiments with respect to the mass of the structure under excitation, and we developed a new method for compensating actuator delay in order to increase the stability condition. In this method, the compensated control signal is generated from the simulation results by using not only displacement but also velocity and acceleration. This method provides a stability criterion (allowable ratio of mass of the structure under excitation to that of a numerical model) about three times larger than that from the current method.