Buoyancy has been found to have a significant influence on the flow and heat transfer behaviors of turbulent sCO(2). This paper uses the computational method to investigate the flow and heat transfer characteristics of turbulent sCO(2) in a large horizontal tube with the buoyancy effects taken into account, RNG and three selected low-Reynolds number k-epsilon turbulence models have been validated against experiments published in literatures. Using the validated CFD model, the buoyancy mechanisms affecting sCO(2) flow and heat transfer within large horizontal tubes are revealed, and the effect of the heat flux have been analyzed. Comparison against experimental results suggests that AEN low-Reynolds number model exhibits the best prediction. Buoyancy influences the flow structure and turbulence levels mainly via the induced secondary circulation. Buoyancy effects are stronger at increased heat flux values. The secondary circulation becomes pronounced at higher heat flux levels and increases the temperature difference between the top and bottom tube surfaces. In large horizontal tubes, slight heat transfer enhancements in the mixed convection are observed near the pseudocritical point. However, a significant deterioration is found at higher heat load density. This is a result contrary to past reports confined to small diameter tubes.