Nowadays, heavy peak-shaving task requires ultra-supercritical boilers having fast and flexible response-characteristic. Wide operating loads and quick load-change mode ask for better flow and heat transfer characteristics of the water wall. Under the above backgrounds, heat transfer to water flowing upward in an inclined smooth bare tube was experimentally investigated under the operating conditions of an ultra-supercritical boiler. The experimental results show that the metal temperature of the tested water wall tube is safe from 30% to 90% BMCR. The heat transfer coefficient of single-phase water at bottom side is 1–2 times bigger than that at the top. The Fr number put forward by Kefer et al. can give a convincing prediction on the effect of flow stratification in the tested pressure range 12–18 MPa. The calculated steam quality difference with the Kefer et al. correlation is an average-value prediction on axial length of the transitional region. At supercritical pressures, four widely used heat transfer correlations are evaluated with the experimental data. The comparison results demonstrate that Swenson et al. correlation gives the best prediction on heat transfer. The effect of flow stratification is evaluated by the modified correlation of Petukhov and Popov, which demonstrates that the effect of flow stratification can be neglected in the high enthalpy region.