Laser cutting tool is promising equipment to avoid from any danger while decommissioning nuclear power plants. Assist gas is used in the laser cutting tool for maximizing the performance of laser by eliminating molten metal. Despite of relatively low use of rectangular nozzle, it is expected that nozzle with this shape has advantageous aspects of mass flow and cutting efficiency in terms of having similar to the cut surface. In this study, comparison and analysis of the nozzle with existing sonic nozzle and supersonic Laval nozzle were performed using inviscid supersonic nozzle theory. In addition, comparing and analyzing minimum length nozzle with a large aspect ratio and the nozzle with square-edged tip, we observed different results at the slit for each case. According to the results, we were able to confirm the superiority of supersonic minimum length nozzle with a large aspect ratio comparing to square-edged nozzle tip geometry.