The experimental system for the transcritical CO2 residential air-conditioning with an internal heat exchanger was built and the effects of working conditions on the system coefficient of performance (COP) were investigated. The results show that evaporator side air inlet parameters have little effects on COP; the increasing range of COP is lower than 6% and 4% in the studied evaporator side air inlet temperature and velocity regions, respectively. The gas cooler side air inlet parameters and system evaporating temperature are the main influencing factors for COP. The COP reduces about 20% when gas cooler side air inlet temperature increases from 32.5 degrees C to 37 degrees C, augments about 27% when gas cooler side air inlet velocity increases from 0.68 m/s to 1.8 m/s and augments about 11% when evaporating temperature increases from 8.7 degrees C to 13.9 degrees C. Then based on the experimental data, the throttling loss and its effect on COP were analyzed. Although the internal heat exchanger can reduce the throttling loss of the transcritical cycle, there is still 5% cooling capacity loss in the throttling process, which causes the system COP loss about 20%. The cooling performance of the gas cooler directly influences the throttling loss, the better the cooling performance the smaller the throttling loss. And the throttling loss reduces with the increase of evaporating temperature. At last, some suggestions were provided for next research. (C) 2009 Elsevier Ltd. All rights reserved.