The application of fin-and-tube heat exchangers with small diameter microgroove tubes in room air conditioners has rapidly increased recently, and fins suitable for such heat exchangers should be developed. In the present study, a louver fin suitable for the evaporator with small diameter microgroove tubes was designed by applying computational fluid dynamics method, and the performance of the optimized and existing louver fins were experimentally investigated. Experimental results show that water bridge occurs at the bottom of the optimized fins for small diameter tubes, which is not the same as that for larger diameter tubes; for the fins of the evaporator with small diameter tubes, the increase of the fin pitch leads to the decrease of Colburn j factors. Based on the experimental data, a correlation of the Colburn j factor was developed to predict the heat transfer rate of the evaporator with small diameter microgroove tubes, and agrees with 85% of the experimental data within a deviation of ±20%.