This paper presents the characteristics of power generation output at Haeng-won wind farm and how to determine the optimized ESS capacity for power stabilizing. Depend on the fluctuation rate of wind power output variation, wind farm capacity and site, power stabilization will be impacted. Therefore, we need to determine proper ESS capacity. Using the actual data of Haeng-won wind farm from 2009. 3 to 2010.2., capacity of ESS was determined by moving average value. To verify the proposed algorithm, simulations are carried out with PSCAD/EMTDC program. As a result, optimal ESS capacity of Haeng-won wind farm in Jeju is estimated about 1.63 MWh.