The current work focused on determining the friction factors in five rectangular channels with hydraulic diameters varying from 69.5 to 304.7 μm and with aspect ratios changing from 0.09 to 0.24. The single-channel test sections were carefully designed such that the friction factors could be determined accurately from the experimental data. R134a liquid and vapor were used as the testing fluids. During the experiments, the Reynolds numbers were varied between 112 and 9180. The measured friction factors were compared with the conventional correlations. The results support such a general agreement in the literature that the flow friction in microchannels may be different from the conventional results. However, a more important finding is that when the channel surface roughness was low, even for the smallest channel tested, both the laminar friction factor and the critical Reynolds number approach the conventional values. In the turbulent region, the surface roughness has great effect on the flow friction even for the smoothest channel tested (Ra/D h = 0.14%).