The characteristics of carbon dioxide cooled under supercritical condition were investigated theoretically and experimentally. Numerical calculations were conducted to get a full understanding of the heat transfer characteristics under variable properties conditions. The effects of parameters such as mass flux, pressure, heat flux and tube diameter on heat transfer coefficient and pressure drop were calculated, and the results were discussed from the distribution of thermodynamic properties inside the cross section. A serial of experimental measurements with a pump cycle was conducted to confirm the above results. Large amounts of experimental data were obtained. These results were compared with those of numerical calculation, and a good agreement was found. The numerical and experimental results show that mass flux and pressure affect the heat transfer coefficient and pressure drop greatly for both middle and small sized tubes, and the effect of heat flux can be observed significantly for a middle sized tube, but for a small sized tube, the temperature distribution inside the cross section is usually small enough to make the effect of heat flux ignorable.