Abstract: Comparison on kinetics of dehydration reaction of pure gypsum and phosphogypsum was studied. Dehydration process of pure gypsum and phosphogypsum was tested using synchronous thermal analyzer. Kinetics parameters were calculated using thermogravimetric datas. It was found that dehydration reaction of both phosphogypsum and pure gypsum involved 2 stages. Product of the first stage was CaSO4·0.5H2O, and that of the second stage was CaSO4. Avrami-Erofeev equation could be used to describe the 2 stages of the dehydration reaction. The dehydration rate was controlled by the rate of new crystal formation and growth. For the dehydration of phosphogypsum, values of activation energy of the first stage was 97.67 kJ·mol-1, and that of the second was 92.63 kJ·mol-1. For the dehydration of pure gypsum, values of activation energy of the first stage was 94.56 kJ·mol-1, and that of the second was 79.05 kJ·mol-1.