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This study presented a numerical simulation of heat transfer characteristics in a microchannel evaporator for a transcritical CO2 air conditioning system, and the simulated results have been validated by the experimental data. The evaporator consists of 6 passes with 29 flat tubes. Each flat tube has 10 microchannels having a 1.2 × 0.6 mm cross-section. The two-phase flow was simulated in the range of evaporation temperatures from 5 to 15 °C; the mass flow rates of 24.16, 30, and 34.96 g/s; and inlet vapor qualities of 0.5, 0.61, and 0.75, respectively. The results show that the two-phase flow with the evaporative temperature at the inlet of 5, 10, and 15 °C became superheated at the 3rd, the 4th, and the 5th pass, respectively. At the evaporation temperature of 15 °C, the heat transfer coefficient decreases from 8.1 to 3.6 kW/m2 K as the vapor quality increases from 0.75 to 1. With the mass flow rate of 30 g/s, in the tube length range from 0 to 1.1 m, the heat flux achieved around 1.55 kW/m2. The numerical cooling capacity gets the maximum value of 2.85 kW at the evaporative temperature of 10 °C. The numerical results are in good agreement with those obtained from the experimental results and the other published results, with a deviation of less than 10%. [ABSTRACT FROM AUTHOR]