The system pressure effect on heat transfer of supercritical water (SCW) flows in a horizontal round tube has been studied by using computational fluid dynamics (CFD) technique, aiming for extending previous researches on the buoyancy effect by further investigating the coupling effects of the system pressure and the buoyancy. A commercial CFD software STAR-CD v4.02 has been used for this purpose. Simulation starts with the sensitive study of key issues, i.e. the mesh dependency, the turbulence model influence, and the near-wall treatments. It was found that on baseline mesh of 477 000 elements with near-wall grid resolution of y(+) = 0.2, the simulation using the Speziale nonlinear high Reynolds k-epsilon turbulence model and the Hassid and Poreh near-wall treatment gives the best predictions in comparison with the experimental data. After the validation, further simulations continued to study the system pressure effect on heat transfer characteristics of SCW flows in a horizontal round tube. It was found that when the buoyancy effect is negligible, the system pressure change has significant effects on the heat transfer of the flow. This implied that the SCW physical property variations due to the system pressure change could play some dominate roles on the heat transfer. However, when the buoyancy effect was considerably strong, the system pressure change has less effect on the heat transfer due to the strong influences of the buoyancy force. This finding has indicated that the heat transfer of SCW flows in a horizontal round tube was primarily governed by the buoyancy effect as observed by previous researchers, but the system pressure changes could also have some effects that cannot be simply ignored. (C) 2008 Elsevier Ltd. All rights reserved.