In this paper, battery energy storage systems (BESSs) are integrated into wind farms (WFs) to mitigate the wind power fluctuations. This paper presents a formulation to optimize the operation schedule of the BESS, with the objective of minimizing the energy of the fluctuating component extracted from the output power of the BESS-integrated WF. The optimal operation schedule of the BESS is utilized to generate the charging/discharging instructions for the BESS to enhance the performances on mitigating the wind power fluctuations. Without proper control, the BESS switches between the charging and discharging states frequently and, thus, degrading batteries significantly, which results in a high operation cost of the BESS. To address this issue, the BESS is divided into two parts, which implement charging and discharging instructions, respectively. Sequential Monte Carlo simulation is applied to simulate the operation of the BESS-integrated WF on a typical day and the simulation results are used to evaluate the technical and economic performances on mitigating the fluctuations. The simulation results validate that the solution can mitigate the output power fluctuations with better performance and a less operation cost than the existing solutions.