Frost on heat exchanger fin surfaces increases the thermal resistance and blocks the air flow passages, which both reduce the system energy efficiency. Therefore, investigations of frost formation, especially simulations of frosting on the heat exchanger surfaces are essential for designing heat exchangers that operate with frosting. In this paper, the frost growth and densification processes on fin-and-tube heat exchanger surfaces are numerically investigated using a mass transfer model implemented as a UDF in FLUENT. The model predicts the frost distributions on the heat exchanger surfaces, the temperature distributions and the air flow pressure drop. The results show that the frost is thicker and the frost density is higher on the fin surfaces on the windward side near the tubes while the frost is thinner and the density is lower near the inlet. Very little frost appears in the tube wake region. Frost on the fin-and-tube heat exchanger surfaces restricts the airflow and about doubles the pressure drop after frosting for 50 min. The simulated frost distributions and pressure drops are in good agreement with experimental data, which means that the frosting model can be used to predict frost layer growth on heat exchanger surfaces and predict the resulting airflow resistance.