The diameter effect on the heat transfer of supercritical water (SCW) flows in horizontal round tubes has been studied using computational fluid dynamics (CFD) technique. The numerical simulations are carried out by the STAR-CD solver combined with the user developed subroutines that control the numerical calculation procedures. Through the tests it is found that the discretization scheme using CD, LUD or MARS will not affect the accuracy of the numerical simulations. Through the diameter effect studies, it is found that the heat transfer of supercritical water flows in the horizontal round tube is strongly affected by the buoyancy especially for the large diameter tube. The large diameter (D = 10 mm) tube will have a high risk to have the strong heat transfer deterioration that can introduce a 180 °C wall temperature difference between the top and bottom surfaces due to the buoyancy effects. The magnitudes of the effects can be quantificationally expressed by a ratio of Grashof number over Reynolds number square. Under the high mass flux regime, the heat transfer deterioration will disappear for all the diameters from 5 mm to 10 mm. The different secondary flow patterns at different tube diameters are also studied.