The kinetics of the thermal decompositions of mechanically activated pyrite and non-activated pyrite was studied by using Friedman method at different heating rates of 2.5, 5, 7.5 and 15 K min(-1) in argon. Results show that the activation energies (E), reaction orders (n) and pre-exponential factors (A) are 268.612 kJ mol(-1), 0.42 and 1,094 x 10(15) min(-1) for non-activated pyrite, 243.725 kJ mol(-1), 0.62 and 2.008 x 10(13) min(-1) for pyrite mechanically activated for 20 min (abbreviated as activated pyrite 1, 177.288 kJ mol(-1), 0.65 and 5.924 x 10(9) min(-1) for pyrite mechanically activated for 40 min (abbreviated as activated pyrite 2, respectively. Comparing the integral width of X-ray diffraction peaks (220) and (440) of activated pyrites at different grinding time with that of non-activated pyrite, several values of the deformation (epsilon) of crystal lattice and the particle size (D) of the crystallite were obtained. It is found that the mechanical activation of the original material brings about a change in surface structure of pyrite, which results in non-activated pyrite's transformation into metastable pyrite in the course of mechanical activation. The metastable pyrite is more liable to thermal decomposition than non-activated pyrite.