A flow and heat transfer numerical simulation was performed for a 2D compressible gas flow through a microchannel in the slip regime to investigate the effects of wall roughness. The wall roughness is simulated by rectangular microelements. This effect is examined for gas flows under inlet Mach number ranging from 0.0055 to 0.202. The numerical results demonstrate that the roughness elements have a significant impact on the flow characteristics. For rarefied gases, it is found that roughness effect leads to an increase in the Poiseuille number with increasing roughness height and decreasing element spacing. The surface roughness has a more significant effect on the flow with a lower inlet Kn. Compressible gas flow is also sensitive to the height of the wall roughness elements. In addition, an increase of the relative roughness height leads to a pronounced decrease in the local heat flux for both rarefied and compressible flow. The average Nusselt numbers have a much more significant reduction for a rarefied flow than a compressible flow. The influence of wall roughness on the average heat transfer rate is smaller than that on the Poiseuille number. (c) 2005 Elsevier Ltd. All rights reserved.