During the operation of an ambient air vaporizer (AAV) designed for vaporizing liquefied natural gas (LNG), the condensation of water vapor results in frost accumulation on the surface of the vaporizer tubes. The frost deposited on the surface lowers the heat transfer rate because the thermal conductivity of frost is less than 1/40 times the thermal conductivity of aluminum, which was used as the material for the tube and fin. It is essential to understand the behavior of frost accumulation because the actual challenge in this field is to predict frost growth in cryogenic conditions and the dynamic heat transfer performance of the AAV owing to frost. In this study the formation of a frost layer on the surface of an LNG vaporizer and the reduction in heat performance are evaluated using a dynamic numerical model that combines heat transfer and mass transfer. The trend of frost growth and the temperature profile of LNG with position and time were studied by modeling and simulation. The numerical results predicted by the model are validated by the experimental data of a pilot-scale AAV, and the mean absolute error (MAE) between the experimental data and model-predicted values was 5.5%. The water vapor condensed by cryogenic LNG in the vicinity of the inlet augments the frost and increases the heat transfer resistance. Under the experimental conditions, when 1 to 2 mm of frost accumulates every 2 h, sensible heat flux is reduced by 15% after 6 h. It is observed from the growth tendency of the frost that the region where LNG is vaporized shifts to the back of the tube over time. Sensitivity analysis of air conditions such as relative humidity, temperature, and velocity presents guidelines for selecting the plant location, climatic conditions, and capacity of fans.