This paper presents an evaluation method of operation performance of a HVAC system based on the ideal operation level of system. The HVAC system is divided into three subsystems and the ideal exergy flow model of each subsystem is set up based on exergy analysis. The ideal operation level of subsystem is defined and obtained by minimizing avoidable part of exergy destruction caused by the control condition. Based on the ideal operation levels of three HVAC subsystems, the ideal operation level of the HVAC system is set up. The evaluation index-improvement potential (IPsys) is defined to evaluate the operation performance of the HVAC system based on the ideal operation level. Five control strategies are applied in the HVAC system to validate the evaluation index and the evaluation method. The results show that the daily IPsys of using the global optimization control strategy on a typical day is 0.115 and is the smallest among those five strategies. It implies that the operation performance of the HVAC system with the global optimization control strategy is the closest to the idea operation level. The evaluation results indicate that the evaluation method not only can evaluate the operation performance of the HVAC system, but also can give the direction for improving the operation performance of the HVAC system. (C) 2015 Elsevier B.V. All rights reserved.