The European Union (EU) banned R404A in new commercial refrigeration systems from 2020 due to its higher global warming potential. One of the alternatives to R404A that can be used in refrigeration systems is R454C. R454C is a zeotropic blend refrigerant consisting of R32 and R1234yf (21.5% and 78.5% by mass) with a GWP of 148. However, since R454C is a mildly flammable refrigerant, the refrigerant charge must be reduced to minimize the risk of flammability. Therefore, to satisfy this concern, a small-diameter microfin tube is used. In this study, a specially designed test section was used. The test tube is a microfin tube with 3.5 mm outside diameter. The test section is divided into three subsections with total length of 852 mm. The temperature of the wall of the test section is measured by 24 T-type thermocouples placed at the top and bottom of the tube. This study investigated the condensation and evaporation heat transfer coefficient of R454C, which was carried out at saturation temperatures of 20 °C, vapor quality between 0 and 0.9, and mass velocity of 100 and 200 kg m−2 s−1. The effects of mass velocity, vapor quality, saturation temperature, and heat flux on condensation and evaporation were presented.