A capacity determination method of an energy storage system（ESS）for smoothing the fluctuations of tie-line power flow in a grid-connected microgrid（MG）with high penetration of renewable energy sources（RES）is presented.The proposed method can determine the appropriate ESS capacity to inhibit the adverse effects of RESs fluctuations on the MG and power grid.Meanwhile，the spectrum analysis of the controllable power needed by the MG for stabilization of tie-line power flow is introduced and analyzed. Based on the spectrum analysis，the minimum ESS power and energy capacity can be determined after given the data of RES output，load，rated power of controllable power sources and the target output of tie-line power flow in the MG.The results of case study prove the method proposed is effective.