The aim of this work is to make a detailed study of the drying kinetics of diammonium phosphate (DAP) grains disposed in a fixed bed in order to determine the mechanisms restricting the transfer of water vapor in the grain. The drying rate is measured using a microbalance. The solid sample consisting of a pile of spherical grains of the humid DAP is placed on the scale and is subject to a constant flux of heat. Theoretical analysis of the matter’s internal transfer is carried out using a vapor diffusion model. The temperature gradient within the granules and the external mass transfer resistance are neglected. Experiments show that the drying rate depends enormously on the temperature and is inversely proportional to the radius squared. The agreement between the calculated results and the laboratory measurements is very satisfactory. The postulated mechanism is thus fully compatible with the observations.