As increasingly installation of wind turbine into the power systems, both academics and industrial specialists have paid a great intention to mitigate fluctuations caused by intermittency nature of wind power generation. Among them, large scale stationary battery system is one of the main means for compensating wind power fluctuations, and many utilities in Japan have set their technical requirements for mitigating fluctuations. In this paper, Output Fluctuation Mitigated Type (technical requirements by Tohoku Electric Power Co., Inc.) is considered as the control target, and a hybrid wind/battery power system is investigated to optimize the capacity of the battery system. A control algorithm for mitigating fluctuation and optimize battery capacity is proposed, and verified by the simulations results, which shows that the battery capacity for the studied system is optimized.