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 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 micro-fin tube and compared with the data of a plain tube. To ensure the data reliability, the micro-fin tube data were also compared with the existing correlations. From the micro-tube results, the heat transfer and friction factor characteristics from laminar to turbulent were clearly established. The transition from laminar to turbulent was found to be inlet dependent. Furthermore, it was observed that the buoyancy effect was present in the laminar region. Based on the heat transfer and friction factor characteristics, the correlations for micro-fin tube were developed by the empirical regression method. The efficiency index (the ratio of the heat transfer and the friction factor of micro-fin tube to those variables for the plain 5 tube) was examined and it was found to have a value larger than one when Reynolds number was larger than 5000 regardless of the type of inlet configuration used. Based on the experimental results, it is recommended that the micro-fin tube should not be used when the Reynolds number is less than 5000 since the efficiency index is less than one.