In this study, the effect of the geometry of the micro asymmetric converging-diverging nozzle was evaluated based on the numerical simulation for the supersonic air flow inside the micro-channel. This nozzle was designed to generate low temperature air flow for the high heat flux heat sink and configured by simple arc curvature. The channel length was 10 mm. The nozzle width was varied from 20 to 400 μm. The inlet pressure, outlet pressure, and wall temperature were 0.7 MPaA, 0.1 MPaA, and 350 K, respectively. Then, in order to evaluate the effect of back pressure, outlet pressure was varied from 0.1 to 0.4 MPa for the case of 200 μm of nozzle width. From calculation results, the pressure and temperature distributions inside the each channel were compared and discussed.