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 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 pressure drop and heat transfer from laminar to transition and eventually the turbulent regions. Therefore, more in-depth study is necessary. In this study, pressure drop and heat transfer were measured simultaneously in a single test section fitted with several micro-fin tubes and the measured data was compared with the data of a plain tube. There were different fin geometries (fin spiral angle, fin height, and number of fins per cross-sectional area) inside the micro-fin tubes. From the friction factor results, the transition from laminar to turbulent was clearly established and shown to be inlet dependent. The transition friction factor characteristic for the micro-fin tubes was different from that of the plain tube. The transition range for the micro-fin tubes was shown to be much wider than that of the plain tube. From the experimental results, it could also be observed that the increase of fin spiral angle lead to the early transition for the friction factor. For the heating condition, the effect of heating on the friction factor was observed primarily in the lower transition region. From the heat transfer results, the transition from laminar to turbulent was clearly established and shown to be inlet and spiral angle dependent. The larger spiral angle caused the earlier transition and the higher heat transfer inside the micro-fin tube. For all the micro-fin tubes with two inlet types, it can be observed that the efficiency index is larger than one when Reynolds number is larger than 5,000.