An experimental investigation on the heat transfer characteristics of CO2 during gas cooling process in a horizontal tube is conducted. The experimental data is obtained over a mass flux range of 79.6-358.1 kg·m-2·s-1

inlet pressure range of 7.5-9.0 MPa and mean bulk temperature from 25.0 to 50.0℃. The effects of mass flux

pressure and bulk temperature on the heat transfer efficiency are investigated. The combined parameter of Gr/Re2 is used to quantify the buoyancy force effect on the heat transfer. The experimental results show that the heat transfer coefficient of the CO2 increases with increasing mass flux. The peak value of the heat transfer coefficient shifts to a higher temperature region as the pressure increases. It appears that the peak value of the heat transfer coefficient occurs at bulk temperature slightly lower than the pesudo-critical temperature at low mass flux. The lower mass flux is reached

the greater influence of buoyancy force effect. The experimental heat transfer coefficients are compared with some existing correlations. The predicted results present obvious deviation compared to the experimental results at low mass flux. A new heat transfer correlation for the tube is proposed based on the experimental data. The maximum error between the predicted results of the new correlation and the experimental data is 20%.