Abstract: The heat transfer at supercritical pressure is mainly characterized by the thermo-physical properties which vary strongly near the vicinity of pseudo-critical point. The strong variation of density leads to great buoyancy effect at low flow rate, which imposes great influence on supercritical fluid flow and heat transfer. The convection heat transfer deterioration of supercritical water in 10 mm vertical tube was studied, and an improved low Reynolds turbulent model was proposed in numerical study to analyze the heat transfer deterioration phenomenon. The calculation results of supercritical fluid heat transfer show that traditional turbulence models overestimate wall temperature, but the present model can predict wall temperature correctly. The numerical results also show the buoyancy effect on turbulent shear stress and flow velocity distribution, which leads to heat transfer deterioration and recovery.