Abstract: In ultra-supercritical pressure region,the heat transfer characteristics of water in a vertical upward internally ribbed tube with the diameter of Φ28.6×5.8mm have been experimentally investigated.The tests have been performed under various conditions with the pressure ranging from 25 to 35Mpa,the mass velocity from 450 to 1800 kg/（m2.s）,and the internal wall heat flux from 200 to 600 kW/m2.The results show that the heat transfer of ultra-supercritical water is better below the pseudo-critical point than that above the pseudo-critical point.Increasing the mass velocity can improve the heat transfer of ultra-supercritical water with a much stronger effect below the pseudo-critical point than that above the pseudo-critical point.The pressure has only a moderate effect on the heat transfer of ultra-supercritical water when the temperature of water is below the pseudo-critical point.Sharp rising of inner wall temperature near the pseudo-critical region takes place earlier at a higher pressure.For given pressure and mass velocity,the internal wall heat flux also shows a significant effect on the distributing of inner wall temperature.The inner wall temperatures are higher at a higher heat flux.Increasing of internal wall heat flux leads to an early occurrence of sharp rising of the wall temperature.Based on the experiments,correlations of heat transfer coefficients are also presented for the vertical upward internally ribbed tubes.