This paper deals with laser cutting of thin steel plate by pulsed YAG laser using newly designed Laval nozzle. The assist gas flow spouted from the Laval nozzle dose not have Mach Shock Disk in free jet, and its straightness is superior to a traditional convergent nozzle. Therefore, the pressure on workpiece increases, and the dross height can be reduced remarkably even under low supply gas pressure condition. Thus, the consumption of flow quantity of assist gas can be reduced, when the dross height is equal. Besides, color change by excessive oxidation due to the heat conduction from dross is smaller at the back side of workpiece around the kerf, since Laval nozzle can remove melted material more effectively compared to a convergent nozzle. However, the supplied gas pressure should be carefully set, since Laval nozzle has many unstable regions, where the pressure on workpiece changes periodically. The dross height dose not increase or almost equal in the case of Laval nozzle, when the gap distance between the nozzle tip and the workpiece surface is changed from 1mm to 2mm. Because the assist gas flow from Laval nozzle has an excellent straightness and the pressure on workpiece does not reduce drastically. Moreover, it was made clear that Laval nozzle designed for the special condition can reduce the dross more effectively and decrease the unstable region.