This paper provides an experimental investigation of heat transfer and pressure drop of supercritical carbon dioxide cooling in a microchannel heat exchanger. An extruded flat aluminum tube with 37 parallel channels and each channel of 0.5 mm × 0.5 mm cross section was used as the test section. The temperature drops of supercritical CO2 cooled inside the test section were controlled at 2 °C, 4 °C, and 8 °C separately for each test to investigate the effect of property change on the friction and heat transfer performance at various temperature cooling ranges near the critical point. The test results showed that while the test conditions were away from the critical point, both heat transfer and pressure drop performance agreed very well with those predicted by conventional correlations. However, for the test conditions near the critical point, the difference between those of the test results and the predicted values is very high. Both heat transfer and pressure drop were strongly affected by the ranges of temperature cooling in the test section while they were near the critical conditions. Since there is a drastic peak of the property change near the critical point, if we use the properties integrated but not averaged from inlet to the exit temperatures, we obtain the results that agree well with the values predicted by conventional correlations. The heat transfer and pressure drop performance of supercritical carbon dioxide in microchannels with size near 0.5 mm are indeed similar to these at normal conditions if its properties are appropriately evaluated.