The unique characteristics of thermo-physical properties under supercritical pressures have a profound effect on the heat transfer. The buoyancy effect that is induced by the severe variation of density in radial direction would cause the heat transfer deterioration. In this study, numerical simulation is conducted with R245fa at 4MPa to investigate the mechanism on the phenomena. Low-Re turbulence model is adopted for the simulation. Results show that the flow field manifested by the M-shaped velocity profiles is significantly distorted due to the effect of buoyancy, leading to suppression of the turbulence production. Comparison with a previous theoretical study shows good agreement on the location of salient points of velocity profiles. Simulation with the density-modified R245fa is conducted as well. Results show that the heat transfer deterioration is improved with a smaller density difference between liquid-like and gas-like region.