In this study, the decomposition of phosphogypsum at different atmospheres of pure CO, N2 and 10% of CO were investigated to gain knowledge about the thermal decomposition of phosphogypsum. It was found that the starting decomposition temperature is 820°C in pure CO, the temperatures are 1080°C for N2 and 890°C for 10% of CO, respectively. Quantitative XRD method was applied to determine the amounts of CaSO4, CaO and CaS in the residues. The results indicate that CaS was mainly formed during Phosphogypsum decomposition in pure CO. In N2 CaO is the main residue from reactions, accompanying a small amount of CaS, and there are CaO and CaS in the decomposed productions, in 10% of CO. Moreover, The Starink,Kissinger and Flynn-Wall-Ozawa methods were used to calculate the kinetic parameters, respectively. The results have shown that the activation energy is in the range of 290.84-317.34 kJ/mol for in pure CO, 335.98-360.90 kJ/mol for 10% CO, 476.39 kJ/mol for N2.