Abstract: Convection heat transfer during the upward flow of CO₂ at supercritical pressures in a vertical circular tube（d_in=2 mm） at high Reynolds numbers was investigated experimentally,and the effects of heat fluxes,mass fluxes,inlet temperatures,pressures,buoyancy and thermal acceleration on the convection heat transfer was analyzed.The results show that the tube wall temperature occurs abnormally distribution for high heat-fluxes with upward flow.The degree of deteriorated heat transfer increases with increasing heat flux.Increasing of the mass flux delays the occurrence of the deterioration of heat transfer and weakens the deterioration of heat transfer down-stream section.The inlet temperature strongly influences the heat transfer.The deterioration degree of heat transfer decreases with increasing pressure.