Under the heating condition, it is of interest to see whether the friction factor characteristics in mini-tubes are the same as those of macro-tubes, especially, in the complicated transition region. Therefore, an experimental setup was built for this study to measure the mini-tube pressure drop (friction factor) under the uniform wall heat flux boundary condition. In this study, the fully developed pressure drops were measured in a test section fitted with horizontal mini-tubes of 4 mm and 2 mm inner diameters. A straight tube length was used as the inlet configuration. The entire experiment covered the Reynolds number range between 1000 and 5000 (laminar-transition-turbulent regions). For the friction factor results, heating caused a decrease in the laminar and transitional friction factors and higher heat flux applied to the mini-tube delayed the start and end of the transition region. For both diameters, the friction factor data were reasonably predicted by the classical macro-scale correlations in the laminar, transition, and turbulent regions.