Non-uniform microchannels are used in the micro space propulsions and micro gas turbine generators. The performances of these applications are determined primarily by the performance of the microchannel used. Based on the micro-fabrication technology, three-dimensional rectangular cross-section straight-convergent-divergent-straight (SCDS) microchannel was fabricated. Its width in the throat cross-section was 16μm, the depth was 20μm and the contraction ratio was 1.625∶1 For different inlet-to-outlet pressure ratios, the volume flow rate characteristics of nitrogen were measured experimentally. The pressure ratios ranged from 1.0 to 4.0, outlet volume flow rate ranged from 0 to 0.12 mL/s and outlet Reynolds number was less than 350 The gas flow characteristics were analyzed by numerical simulation based on finite-volume method. Good agreements between computational results and experimental data were found. The simulation results indicate that two novel behaviors are different from that at conventional scales. One is the first sonic point position, the other is the critical pressure ratio（the critical pressure ratio is defined as the local-to-inlet pressure ratio , here the local pressure corresponding to the pressure at the first sonic point in the internal flow of SCDS microchannel）. These two novel behaviors should be attributed to the surface effects. Due to the larger surface-to-volume ratio (S/V) for microchannels, the factors related to the surface effects have more impact on the microflow. So the relationships between novel behaviors and the value of S/V are discussed further.