Abstract: Due to its capabilities to provide fast and accurate regulation, the application of the electrochemical energy storage is suitable for frequency modulation in power grid. In order to accurately evaluate the control performance of energy storage resources when participating in AGC control, four control performance indices in terms of the similarity, delay, deviation and contribution indices are proposed by calculating the similarity between the actual output and ideal output curve of energy storage within certain time period. The new indices can not only accurately represent the energy storage regulating performance and effect, but also be adapted for the evaluation of conventional units. Case studies are performed using the actual operation data of both energy storage units and conventional units, such that the validity of the proposed indices are verified by comparison and analysis.