The heat transfer enhancement (HTE) from buoyancy occurs in upward and downward flows at supercritical pressures when the buoyancy parameter, Bo*, is above 8 x 10(-6). Numerical simulations of experiments on buoyancy-enhanced heat transfer of supercritical CO2 in a heated vertical tube with an inner diameter of 2.0 mm at an inlet Reynolds number, Re-in, of 1970 were performed using low Reynolds number turbulence models. The Myong and Kasagi (MK) model quantitatively predicted the buoyancy-enhanced heat transfer. The trade-off between buoyancy effect on the viscous length scale and the redistribution of velocity profiles resulted in similar heat transfer results although the mechanisms on the two aspects were different for upward and downward flows. Heat transfer deterioration (HTD) occurred in upward flow as Rein increased. The supercritical heat transfer in upward flows for various Rein were compared to obtain a better understanding of the buoyancy effect under HTE and HTD conditions.