A penetration of renewable energy sources such as photovoltaic, wind power etc to prevent global warming is become increasing highly. However, a random unpredictable wind power output may cause frequency fluctuation on isolated hybrid wind-diesel power system. This paper proposes design of robust control of smart load to stabilize frequency fluctuation in isolated wind-diesel power system. The structure of the proposed controllers are the first-order PI controller. The robustness of the proposed PI controller is guaranteed by applying an inverse additive perturbation to represent possible unstructured uncertainties in the power system. The proposed PI control parameters are optimized and achieved by a genetic algorithm (GA). Simulation studies have been done to show the performance of the proposed PI controller in isolated hybrid wind-diesel power system against various disturbances and system uncertainties.