Abstract: The in-depth study on heat transfer characteristics of fluids under supercritical pressure is crucial to the core design of supercritical water-cooled reactor (SCWR). Heat transfer experiments of supercritical Freon R134a flowing upward in a circular tube were carried out with pressure of 4.3-4.7 MPa, mass flux of 600-2500 kg/(m²•s) and heat flux of 20-180 kW/m². The heat transfer coefficient is close to the value calculated by Dittus-Boelter correlation in the region far from the pseudo-critical temperature. However, it is much smaller than the value calculated by Dittus-Boelter correlation with larger heat flux in the region approaching pseudo-critical temperature. The heat transfer deterioration occurs at the condition of small mass flux and large heat flux. It is shown that the heat transfer coefficient is of significant relevance with both acceleration effect dimensionless number and buoyancy dimensionless number. The correlation of onset of heat transfer deterioration is proposed based on experimental data. The heat transfer coefficient calculated by Bishop correlation shows a small standard error but an overall overestimate. The value calculated by Jackson correlation shows a small average error but a larger standard error.