This study investigated the effect of geometrical parameters of gas coolers on system performance and the optimal discharge pressure with a developed and experimentally validated numerical model of an air-source transcritical CO2 heat pump water heater system. This model fully considered actual heat transfer process in the gas cooler and characteristics of major components adopted in the prototype system. Simulation results showed a good agreement with experimental results in terms of heat yields, COP and the optimal discharge pressure. Parametric analysis reveals that a larger heat transfer area of the gas cooler increases heat yields but decreases the optimal discharge pressure while other components in the system remain unchanged. In addition, at the same operating condition with the same heat transfer area, heat yields increase with CO2 mass fluxes by reducing tube diameters and flow passages of gas coolers, and beyond certain range of mass fluxes, the optimal discharge pressure increases sharply. As one result, the optimal range of CO2 mass flux in heat exchange tubes should be the range of 155-325 kg m(-2) s(-1) at given condition in this paper. (C) 2013 Elsevier B.V. All rights reserved,