Frost formation on heat exchangers can have a serious impact on their heat transfer performance due to the thermal resistance and pressure drop by the frost layer. On the cooled surface of a heat exchanger, the frost grows with a spatial distribution that is determined by the temperature and humidity profiles of the air and the flow pattern in the heat exchanger. This means that the mass transfer coefficient for frost formation has both temporal and spatial distribution. In the present study, the mass transfer characteristics of frost formation on a plate-fin tube heat exchanger were investigated by using neutron radiography. Neutrons are strongly attenuated by the water molecules in the frost layer. Frost formation and growth on the fin and tube surfaces could be clearly visualized by neutron radiography. The frost formed most rapidly on the fin and tube edges, and gradual growth was also observed in the wake region of the tube. The frost distribution was measured every 5 min, so that the local frost accumulation rate could be estimated from differential images. The spatial and temporal distributions of the local mass transfer coefficient were successfully measured using neutron radiography.