The numerical result of supersonic jet is non-dimensionalized by the nozzle e exit diameter for analysis of jet structure. In recently, the micro-nozzle as like milli-m or micro-m size is used in industrial engineering. In this case, it seems that whether to compose the similarity of jet structure may be cleared so that this paper aims to clear the influence of the nozzle diameter for the structure of underexpanded supersonic jet by numerical analysis. In the numerical calculation, the nozzle diameter decreases from 12 mm and the jet structure, the location and diameter of Mach disk, the sonic line and the boundary layer in nozzle are analyzed in this study.