Abstract: In order to investigate the convective heat transfer at supercritical pressures, experimental research was conducted on heated Freon R134 a flowing upward through an I.D 25 mm vertical circular tube under supercritical pressure condition. The experimental data covers a wide range of conditions. The pressure is at 4.5MPa and 4.7MPa. The mass flow flux varies from 400 to 700 kg/（m²·s） and the heat flux ranges from 30 to 60 k W/m². Both enhanced and deteriorated heat transfer were analyzed and the parametric sensitivity was also carried out. The heat transfer performance was evidently enhanced near the pseudo-critical point. The deteriorated heat transfer appeared at lower mass flow velocity or at higher heat flux under a certain ratio of q/G=0.06 k J/kg. At a mass flux of 500 kg/（m²·s） two types of deteriorated heat transfer were observed in the experiment: the first type appeared at the near entrance region of the tube and existed within different range of fluid inlet temperature; The second type appeared at any section inside the tube than entrance but only within a certain enthalpy range. Heat transfer can be enhanced by increasing mass flow velocity, decreasing heat flux or decreasing pressure, while the variance of the heat transfer deterioration is opposite.