According to Krishnamoorthy et al.[1], pressure drop measurements for horizontal micro-tubes under isothermal condition have been conducted by various researchers in recent years. From their literature review, it was shown that the friction factor in micro-tubes could unanimously be predicted by using macro-scale theory and that there is a need to investigate certain issues like (a) the effect of micro-tube diameter on the transition Reynolds number range and (b) the effect of the inner surface roughness on the friction factor and transition region. Regarding to the point (a), Ghajar et al. [2] measured the pressure drop for a horizontal mini- and microtubes with various diameters in the transition region under isothermal condition. Their experimental results indicated the influence of the tube diameter on the friction factor profile and on the transition Reynolds number range. However, regarding to the point (b), the effect of roughness on friction factor profile and transition was still not fully understood. Moreover, only a few studies have investigated the effect of heating on friction factor in micro-tubes, especially, in the transition region. Therefore, in this study, an experimental setup was built to measure pressure drop for horizontal micro-tubes under the isothermal and uniform wall heat flux boundary conditions. Water was used as the test fluid and the test section was glass and stainless steel micro-tubes with various roughness and diameters. From the measurements, the effect of roughness and heating on friction factor and transition region was clearly observed. For friction factor under isothermal condition, compared to the macro-tube, the micro-tube had a narrower transition region due to the roughness and the decrease in the tube diameter delayed the start of transition. For friction factor under heating condition, the laminar and transition data were different from the isothermal case. Heating also delayed the start of transition. The effect of heating was not seen on the turbulent region. For isothermal and heating boundary conditions, the increase of inner surface roughness induced a narrower transition region. Copyright © 2011 by ASME.