To increase heat transfer, internally micro-fin tubes are widely used in commercial HVAC applications such as flooded evaporators. It is commonly understood that the micro-fin enhances heat transfer but at the same time increases the pressure drop as well. In the previous studies, majority of the works were focused on the development of correlations in a particular flow regime, especially in the turbulent region. There are only a few works that fundamentally studied the continuous change in the characteristic behavior of heat transfer and pressure drop from laminar to transition and eventually the turbulent regions. Therefore, more in-depth study is necessary. In this study, heat transfer and pressure drop were measured simultaneously in a single test section fitted with 2 micro-fin tubes and compared with the data of a plain tube. From the results, the transition from laminar to turbulent was clearly established. The buoyancy effect is present in the laminar region. The transition from laminar to turbulent was found to be inlet dependent. It could be seen that the delay of transition was more obvious for smaller spiral angle while it was not as obvious when large spiral angle tube was used. Furthermore, it was observed that the larger spiral angle had an enhancement of the heat transfer in the upper transition to turbulent regions. Finally, the efficiency index (the ratio of the heat transfer and the friction factor of enhanced tube to those variables for the plain tube) was examined and it was found to have a value larger than one when Reynolds number is larger than 5000 regardless of the type of inlet configuration used. Therefore, the application of the micro-fin tubes used in this study is suitable when Reynolds number is larger than 5000. Copyright © 2011 by ASME.