With the continuously increasing of wind power penetration level in the power system，it is important to alleviate the adverse impacts of wind power fluctuations to the power system operation.As such，the integration of the energy storage system（ESS）at the wind turbine or wind farm level to mitigate the wind power fluctuations becomes an important research topic.This paper describes the existing demonstration projects in which ESS is applied to mitigate wind power fluctuations.Four important issues about the application of ESS are then discussed，including the selection of energy storage technologies，the power and capacity configuration of ESS，the control method of fluctuation mitigation，and the energy management of ESS.Finally，major technical problems and future research topics are discussed and recommended.