Heat transfer of ultra-supercritical water in vertical upward tube under uniform heating was investigated experimentally and numerically. For comparison, similar studies under non-uniform heating were also simulated. The effects of specific heat ratio, buoyancy and acceleration on the heat transfer of ultra-supercritical water were discussed according to the experimental data. Results show that these dimensionless parameters have no single relationship of independence with the heat transfer coefficient which indicates that using them directly to predict heat transfer is inaccurate. The shear stress transport k-co model was used in the numerical simulation. Results concur with the experimental data to a high degree, which proves that the model has strong applicability in predicting heat transfer. The mechanisms of heat transfer enhancement and deterioration were analyzed through simulation results, and the integral of specific heat in the boundary layer and the buoyancy effect were confirmed to be their main factors, respectively. Furthermore, the differences between the uniform and non-uniform heating were also revealed. The wall temperature of smooth tube under non-uniform heating shows a parabola distribution, and the peak value occurs at the midpoint of the heated side. Moreover, the peak value of the wall temperature is higher under non-uniform heating than that under uniform heating; however, the heat flux is higher when heat transfer deterioration occurs. (C) 2018 Elsevier Ltd. All rights reserved.