When the air-source heat pump systems operate at low ambient temperatures in winter, frost forms on the coil surface of the outdoor evaporators. The frost substantially affects the operating performance and energy efficiency of heat pump systems, and hence periodic defrosting is essential. In this study, several defrost methods are presented to look for a candidate for air-source transcritical CO2 heat pump systems. The hot gas method proves to be more suitable among other defrosting methods for transcritical CO2 heat pump systems. To validate its reliability and rationality, an air-source transcritical CO2 heat pump water heater was built in a climatic laboratory. Through the experiments, the dynamic process of temperature and pressure were obtained to demonstrate the hot gas defrosting characteristics and system cycle. The hot gas defrosting cycle in the p-h diagram was also validated by experiment results. Meanwhile, instant defrosting images were captured to record the dynamic defrosting process. The defrosting process lasted 10 min and defrosting efficiency was 34.8% for hot gas defrosting method. The effectiveness and applicability of hot gas defrosting method for CO2 heat pump water heater is validated by experiments. (C) 2014 Elsevier B.V. All rights reserved.