To increase heat transfer, internally micro-fin tubes are widely used in commercial HVAC applications. It is commonly understood that the micro-fin enhances heat transfer in the turbulent region. There are only a few works that fundamentally studied the continuous change in the characteristic behavior of heat transfer from laminar to transition and eventually the turbulent regions. Furthermore, it is difficult to find the information about the effect of inlet configuration on the microfin tube heat transfer in the open literature. Therefore, more in-depth study for the micro-fin tube heat transfer characteristics is necessary. In this study, heat transfer was measured in a single test section fitted with a micro-fin tube and compared with the data of a plain tube. Both of the tubes with the same internal diameter of 14.9 mm were tested under the uniform wall heat flux boundary condition. Three inlet configurations, re-entrant, square-edged, and bell-mouth, were used in this study. The entire experiment covered the Reynolds number range between 1100 and 23,000. From the heat transfer results, the transition from laminar to turbulent region for the plain and micro-fin tubes was clearly established. For both of the plain and micro-fin tubes in the laminar region, buoyancy effects and heat transfer magnitudes were comparable. It was also observed that for the micro-fin tube heat transfer enhancement initiated in the transition region and lasted through the turbulent region. For both of the plain and micro-fin tubes, the transition from laminar to turbulent region was found to be inlet dependent. For bell-mouth inlet, it was obvious that the microfin tube had an earlier transition than the plain tube. Furthermore, for both of the plain and micro-fin tubes with bell-mouth inlet, an unusual behavior of the local heat transfer coefficient was observed in the transition and turbulent regions. Copyright © 2012 by ASME.