Considering interconnection of a large-capacity of wind power generation to the utility grid, it is of great concern that its output power fluctuation has adverse influences, e.g. frequency fluctuation. There have so far been some research works on installation of battery energy storage systems (BESS), as a solution of these problems. However, owing to very high cost of the BESS, its capacity should be as small as possible. In this paper, not only the installation of the BESS as one of measures of suppressing the frequency fluctuation caused by wind power generation, but also blade pitch angle control for blunting the output power of wind turbine generators (WTGs) is also considered. This paper proposes a coordinated control method of the BESS and the blade pitch angle, and evaluates reduction of the capacity of the BESS and the power generation loss caused by blunting the output power which should be originally generated by WTGs.