From the viewpoint of global environmental protection and energy-saving, CO<sub>2 </sub>heat pump water heaters have been developed and commercialized recently. In gas-cooler of CO<sub>2 </sub>heat pump water heater, however, the heat transfer performance of water side deteriorates because of the laminar flow in narrow passage. To clarify the effects of the heat transfer tube geometry on the heat transfer and pressure drop characteristics in water flow passage, five heat transfer tubes of different geometry were tested in this study. The tested tube geometries are as follows: a straight smooth tube (SS), straight tubes with different dimple pitch (SD1, SD2), a twisted smooth tube (TS) and a twisted dimple tube (TD). The pressure drops of twisted tubes (TS and TD) were roughly twice that of the straight smooth tube (SS). The dimpled tubes (SD1 and SD2) heat transfer coefficients increased with increasing in the water Reynolds number. The heat transfer coefficients of twisted tubes (TS and TD) were much higher than that of the straight smooth tube (SS) especially at the water Reynolds number below 1000. The results suggest that applying twisted tube is a promising technique for enhancing the water side heat transfer coefficient.