Convective heat transfer of supercritical CO2 cooled in a vertical circular tube has been numerically investigated by using the computational fluid mechanics code FLUENT. The objectives of this paper are to predict the heat transfer coefficient and to study the supercritical convective heat transfer of CO2 under uniform heat flux. Simulation starts with four cases, including a physical model, mathematical model, mesh independency, and solution method. Based on the mesh size of 80 x 500 in the radial and axial directions with near-wall resolution of less than 1, the low Reynolds number turbulence model proposed by Lam and Bremhorst (LB) model is adopted to investigate the heat flux effect, buoyancy effect on flow direction, and buoyancy effect on fluid flow. The results show that the LB low-Reynolds-number turbulence model agrees well with the experimental data. Buoyancy effect significantly affects the fluid flow and heat transfer characteristics in turbulent flow, which enhances heat transfer in upward flow, and impairs it in downward flow.