Abstract: Characteristics of gas flow in a 2-D Laval micro-nozzle with 37 μm throat-width were studied through numerical simulation based on continuum theory in this paper.Influences of pressure ratio on the configuration of the flow field were investigated in detail.The formation mechanism and requirement of supersonic annular phenomenon,whose essence is "Mach disk" formed in micro-nozzle under over-dilation condition,were analyzed in micro-scale by shock wave theory.While the exit pressure remained unchanged,the possible existent range of supersonic annular phenomenon was judged,and the influence of pressure ratio and shock wave on total pressure drop in the micro-nozzle was discussed.