Cooling heat transfer to supercritical CO2 in a horizontal circular tube has been numerically investigated using CFD code FLUENT in the present study. The purpose is to provide detailed information on heat transfer behavior which is hard to be observed in experimental studies and to help to better understand the heat transfer mechanism. Simulation starts with five key issues, including physical model, mathematical models, mesh independency, boundary conditions and solution methods. The results demonstrate that almost all models are able to reproduce the trend of heat transfer characteristics qualitatively, and LB low Re turbulence model shows the best agreement with the experimental data, followed by standard k-epsilon model with enhanced wall treatment. After the validation, further studies are discussed on velocity and turbulence fields, buoyancy effect, and heat transfer mechanism. It concludes that buoyancy significantly affects the turbulent flow, and evidently enhances the cooling heat transfer of supercritical CO2, especially in the vicinity of pseudo-critical point. The mixed convection is the main heat transfer mechanism during supercritical CO2 cooling process. (C) 2010 Elsevier B.V. All rights reserved.