In this study, we focus on a micro-scale cooling device using a supersonic single phase gas flow. The single phase gas cooling system has advantages for cooling electronics device in a micro-scale. Generally, the forced convective heat transfer by single phase gas flow has a lower heat transfer coefficient than other heat transfer mechanisms. However, the heat transfer rate can be largely improved with a low temperature flow that is generated by isentropic expansion in supersonic nozzle. The objective of this study is to conduct a numerical evaluation of the possibility of this cooling system with a supersonic air flow through a heated micro-fin array. In order to calculate the supersonic flow inside the nozzle and evaluate the effect of the nozzle shape on the heat transfer, two types of nozzles are designed. One nozzle is a typical supersonic nozzle called Laval nozzle. The other is named Bump nozzle which has a simple arc shape at the throat. The channel size of both nozzles are about 200 μm in width and 2743.1 μm in length. In order to estimate the cooling performance, the numerical simulations were conducted by using ANSYS FLUENT 12.1 with the density-based Roe-FDS method. The inlet pressure, outlet pressure, and total pressure were set to 290 kPa, 100 kPa, and 367.1 kPa, respectively. The stagnation temperature and wall temperature were assumed 300 K and 350 K, respectively. The values of bulk mean temperature and Nusselt number were estimated. In both nozzles, the calculated bulk mean temperature was about 230 K and the Nusselt number was 7.54, which is the theoretical value of laminar forced convection between the parallel plates. The results showed that the Bump nozzle had almost the same cooling performance as the Laval nozzle in spite of its simple geometry in the each single channel. In addition, the Bump nozzle can have 4 times the number of channels the Laval nozzle configuration can occupy the same area because of its shape. This indicates that cooling performance of a device that includes the Bump nozzle geometry is higher than that of the Laval nozzle.