Recently, CO2 heat pump water heaters have already been developed and commercialized from the viewpoint of global environmental protection and energy-saving. However, there are still some problems to be solved, and one of the major problems is to improve the heat transfer performance of water flow passage in the gas-cooler. This is because the water flow is laminar. Generally, to enhance the heat transfer performance is difficult in laminar flow. However, dimpling and/or twisting heat transfer tubes etc. are considered to be effective because the water flow is disturbed. In the present study, two kinds of inner tube bundles are tested to clarify the effects of the tube forms on the heat transfer performance at the CO2 flow passage in the gas-cooler experimentally. Pressure drops of the CO 2 flow passage in gas cooler are also evaluated. Tested inner tube bundles made of copper are as follows: (1) a pair of twisted smooth tubes (TS), and (2) a pair of twisted dimple tubes (TD). These inner tube bundles are inserted into a 10.8 mm ID outer tube made of copper, then tubes-in-tube heat exchanger is assembled. The heat transfer coefficients and the pressure drop of CO2 are measured in the following ranges: pressure range is 8 to 10 MPa, mass flow rate range of 19.2 to 34.6 kg/h, and CO2 inlet temperature range of 20 to 70°C. It is found that the heat transfer performance in the CO2 flow passages of both the pair of twisted dimple tubes and the pair of twisted smooth tubes are nearly identical. However, the pressure drop in the CO2 flow passage of the pair of twisted dimple tubes is higher than that of the pair of twisted smooth tubes.