Mixing in microchannels is an important problem because the flow is always laminar flow even though the velocity may be high. The present paper analyzes the inherent factors affecting micro gas mixing using the direct simulation Monte Carlo (DSMC) method at high Knudsen numbers. The discretization errors in the DSMC method were analyzed to ensure numerical accuracy. The simulation results show that the wall characteristics have little effect on the mixing length when the main gas flow velocities for different wall characteristics were the same. The mixing length is nearly inversely proportional to the gas temperature. The dimensionless mixing coefficient, which is the ratio of the mixing length to the channel height, is proportional to the Mach number and inversely proportional to the Knudsen number. This conclusion was validated for microscale gas flows, but is expected to apply to all laminar gas flows at all scales. (c) 2005 Elsevier Ltd. All rights reserved.