ABSTRACT： Integrating a battery energy storage system （BESS） with a wind farm with many wind power units could mitigate the output power fluctuations of the integrated system. Thus, it is an important issue to be addressed to develop an appropriate control strategy of BESS for effectively mitigating the fluctuations of wind power outputs and then maximizing the accommodation capability of the power system concerned for wind power integration. Firstly, the mathematical model of BESS is presented with the structure and characteristics of the integrated system taken into account. An improved control strategy is then proposed with the interactions between the current charging/discharging behavior of BESS and the state of charge （SOC） constraints at the next time slot taken into account, so as to fully make use of the BESSs capacity. And then, an optimal control strategy is developed for minimizing the over-limit probability of combined output power fluctuations from the integrated system, and a highly efficient solver, named AMPL/CPLEX, is employed to solve the optimization model. Finally, an integrated system including a large wind farm and a BESS is employed to demonstrate the performance of the proposed improved control strategy and optimal control strategy. Simulation results show that these two BESS control strategies could effectively mitigate the fluctuations of the combined output power of the integrated system.