An isolated power system with renewable sources is very interesting to be applied in electrical power system due to environmental friendly, reducing cost etc. However, the intermittent of output power of renewable energy may cause power fluctuation of the grid, and it also cause a serious problem of frequency of the grid. This paper proposes design of robust control of battery energy storage to stabilize frequency fluctuation in an isolated power system. To compensate for such power variations of wind power, battery energy storage which is able to supply and absorb active power quickly, can be applied to control the frequency fluctuation. The structure of a controller is the first-order lead-lag compensator. An inverse additive perturbation is used to represent possible unstructured uncertainties in the power system. The genetic algorithm (GA) is employed to solve and achieve the control parameters. Simulation studies have been done to show the control effect and robustness of the proposed controller against various disturbances.