The heat transfer characteristics of supercritical CO2 cooled in the helically coiled tube are investigated experimentally and numerically. The inner diameter, coil pitch and coil radius of the helically coiled tube are 4 mm, 34 mm and 36 mm, respectively. The effects of heat flux, pressure and mass flux on the heat transfer are analyzed based on 512 sets of experimental data, a new modified correlation is developed to calculate the heat transfer coefficients of supercritical CO2 in the cooled helically coiled tube. To study the buoyancy effect, the heat transfer coefficients of upward flow are compared to that for horizontal flow, and it is found that the effect of buoyancy is negligible in the liquid-like region, but significant in other region under the experimental conditions. On this basis, the three buoyancy parameters: Bu-p, Gr(th)/Gr(q) and Ri are applied to predict the effect of buoyancy on the heat transfer. The three parameters overestimate the impact of buoyancy. Numerical analysis of the cooling heat transfer coefficient in helically coiled tube is conducted by using shear-stress transport (SST) model. The flow fields are analyzed and the effect of heat flux is mainly related to the distribution of specific heat (c(p)) in the radial direction of tube transverse section. (C) 2017 Elsevier Ltd. All rights reserved.