Abstract: Within the range of pressures from 9 to 28MPa, mass velocity from 600 to 1200kg/（m²·s）, and internal heat flux from 200 to 600kW/ m², experiments have been performed to investigate the heat transfer characteristics of sub-critical and supercritical steam-liquid in an inclined upward internally ribbed tube with the inclined angle of 19.5 degree from the horizon, the outer diameter of 38.1mm, and the thickness of 7.5mm. Based on results, it was found that heat transfer enhancement of the internally ribbed tube can restrain the departure from nucleate boiling at sub-critical pressure. Near the critical pressure, heat transfer enhancement of the internally ribbed tube decreased. At supercritical pressure, the temperature differences between the wall and the fluid were small. Enough mass velocity needs to be supplied in the practical design. Correlation of critical mass velocity has been provided.