The existing experimental data in the literature on friction factor in microchannels are analyzed. Flow characteristics for nitrogen and helium in stainless steel microtubes, fused silica microtubes and fused silica square microchannels are studied experimentally. The data in fused silica microtubes with diameters ranging from 50 to 201 mu m and the data in fused silica square channels with hydraulic diameter ranging from 52 to 100 mu m show that the friction factors are in good agreement with the theoretical predictions for con ventional-size channels. The friction factors in stainless steel tubes (D = 119-300 pm) are much higher than the theoretical predictions for tubes of conventional size. This discrepancy is resulted from the large relative surface roughness in the stainless steel tubes. From the literature review and the present test data it is suggested that for gaseous flow in microchannels with a relative surface roughness less than 1%, the conventional laminar prediction should still be applied. A positive deviation of the friction factor from the conventional theory is observed due to the compressibility effect. In addition, smaller friction factors in fused silica microtubes with inner diameters ranging from 10 to 20 mu m are obtained and the decrease in friction factor from the rarefaction effect is observed. (c) 2006 Elsevier Ltd. All rights reserved.