Experimental studies on heat transfer to supercritical water flowing upward in circular tubes are carried out at the SWAMUP test facility. Totally more than 10,000 test data were obtained in tubes of two different inner diameters, i.e. 10.0mm and 7.6mm with the following test conditions: pressure from 23.0MPa to 26.0MPa, mass flux from 450kg/m 2 s to 1500kg/m 2 s, heat flux from 190kW/m 2 to 1400kW/m 2 and bulk temperature from 240°C to 460°C. Each test point is accompanied with systematic error analysis. It is found that for 90% of the test data the uncertainty of measured Nusselt number is less than 10%. Based on the test data obtained, the effect of various parameters on heat transfer is investigated, such pressure, mass flux, heat flux and tube diameter. In addition, the importance of several dimensionless numbers and the accuracy of five widely applied heat transfer correlations are assessed. It is concluded that the acceleration number π A and the buoyancy parameter π B show unique effect on heat transfer coefficient. Among the five selected heat transfer correlations, the correlation of Cheng et al. and Jackson show the best agreement with the present test data.