Within the range of pressure from 9 to 28 MPa, mass flux from 600 to 1500 kg/m2s, heat flux at inside wall from 200 to 600 kW/m2, and wall temperature up to 650 °C, experiments were conducted to research the forced convection heat transfer of water in an inclined upward tube with an inclination angle of 20 deg and an inner diameter of 26 mm. According to the experimental data, the effects of pressure and heat flux on heat transfer of water were analyzed in detail. In the subcritical pressure region, it was found that heat transfer characteristics of water are not uniform along the circumference of the inclined tube. Temperature of the top is always higher than that of the bottom, which can be attributed to the buoyancy effect in the inclined tube. In the supercritical pressure region, natural convection makes the low-density hot fluid gather at the top of the inclined tube; hence, heat transfer condition is deteriorated and wall temperature is increased. Furthermore, the criterions of Petukhov and Jackson were selected to judge the buoyancy effect in the inclined upward tube. The result seems acceptable but these criterions should be further improved to get a better applicability for an inclined tube.