The application of carbon dioxide in heat pump systems has seen increasing interest in recent years because of its particular temperature glide in the super-critical condition and ecological sustainability. Together with microchannel heat exchangers, carbon dioxide has been demonstrated to have better heat transfer performances. However, the performance of carbon dioxide microchannel heat exchanger has a close connection with header designs. With a view to investigating the influences of header designs of heat exchangers on the performance of the heat pump system, experimental studies of carbon dioxide microchannel heat exchangers with different headers were conducted in this communication. Experimental tests were implemented under two different discharge pressures of the compressor and two different air flow rates to observe relationships among refrigerant flow rates, heat transfer rates, pressure drops of heat exchangers and system COP.