In this study, an experimental study on the heating performance of a CO2 heat pump water heater was performed with a variation of operating conditions such as refrigerant charge amount, outdoor temperature, compressor frequency, EEV opening and water mass flow rate. Based on the test results, the optimum charge amount was 1800 g. At the water mass flow rates of 75, 85, and 95 kg/hr, the water heating temperatures were 62, 67, and 74°C and COPs were 2.6, 2.8, and 3.0, respectively. Besides, the water mass flow rate and compressor frequency were varied to maintain the water heating temperature above 60°C with the decrease of outdoor temperature. Thus, the compressor frequency increased beyond 60 Hz and the water mass flow rate was 45 kg/hr at the outdoor temperature of-13°C, 65 kg/hr at-8°C, 75 kg/hr at-3°C, and 85 kg/hr at 2, 7°C. As the outdoor temperature decreased, the COP decreased by 2.5-39.8%.