In power systems, the engine generator plays an important role such as providing power at the time of power failure and at the stand-alone operating system. In using the engine generator for stand-alone system, there are upper and lower limits in the engine speed variation. As a result, if the engine speed exceeds the value, the engine generator comes to unstable and cannot go on the operation. So when we place a load on engine generator, we have to take the engine speed variation limit into consideration, as a result, the amount of the load come to be limited. In this research, we attempt to reduce the deviation of engine speed during the transient by using the energy storage system (ESS) which is used in the system of inverter control. In this paper, control scheme of ESS is shown and the simulation and experimental result are also shown with comparison between simulation and experiment as a result.