The in-tube condensation heat transfer characteristics of CO2 in three different microchannels were investigated. The channels were rectangular, and the numbers of ports were 7, 19, and 23. The hydraulic diameters were 1.5, 0.78, and 0.68 mm for the 7, 23, and 19 ports, respectively. The test temperature ranged from -5 to 5 degrees C, and the range of the mass flux was from 400 to 800 kg/m(2) s. The condensation heat transfer coefficient increased with the decrease in hydraulic diameter, which is dominant at condensation temperatures of 0 and -5 degrees C. Increasing and decreasing the heat transfer coefficient at critical vapor quality was observed for the microchannel of 23 ports. The highest pressure drop was found in the microchannel of 23 ports too. The model by Thome et al. showed the smallest mean deviation of heat transfer coefficient, and the Mishima and Hibiki model had a mean deviation of +/- 30.1% for the frictional pressure drop. (C) 2013 Elsevier Ltd. All rights reserved.