Considering the complementary characteristics of energy storage system（ESS）and thermal power unit on the automatic generation control（AGC）regulation，based on the AGC operational experience in North China Grid，an optimization method for allocating AGC capacity considering ESS is proposed.The relationship between capacity ratio of ESSs and thermal power units and overall system AGC performance are analyzed using AGC simulation tools.Based on the designated multiple AGC objectives at the system level，a modelling method for optimizing the AGC capacity of different types of AGC resources using a linear programming model is proposed as well.Finally，a simplified system including 5 thermal power units and 2 ESSs is used in the case study.Results of the case study indicate that appropriate utilization of ESS will better assist power system to realize the AGC optimal operation technically and economically.