An effective frost thermal conductivity for the crystal growth period of frost formation was developed based on a simple model consisting of cylindrical frost columns surrounded by moist air. Being closely related to the frost layer density, the thermal conductivity of frost layer is found to be affected by the vapor diffusion inside the frost layer as well. The density of frost columns, on the other hand, depends on the sublimation temperature of ice crystals and the humidity ratio of air, therefore, the whole process of frost formation and the thermal conductivity of frost layer, in particular, is shown to depend on the controlling temperatures, i.e. plate temperature, air temperature, and air humidity ratio.