The thermal decompositions of mechanically activated and non-activated pyrites were studied by thermogravimetry (TG) at the heating rate of 10 K min −1 in argon. Results indicate that the initial temperature of thermal decomposition ( T di) in TG curves for mechanically activated pyrites decreases gradually with increasing the grinding time. The specific granulometric surface area ( S G), the structural disorder of mechanically activated pyrites were analyzed by X-ray diffraction laser particle size analyzer, and X-ray powder diffraction analysis (XRD), respectively. The results show that the S G of mechanically activated pyrites remains almost constant after a certain grinding time, and lattice distortions ( ε) rise but the crystallite sizes ( D) decrease with increasing the grinding time. All these results imply that the decrease of T di in TG curves of mechanically activated pyrites is mainly caused by the increase of lattice distortions ε and the decrease of the crystallite sizes D of mechanically activated pyrite with increasing the grinding time. The differences in the reactivity between non-activated and mechanically activated pyrites were observed using characterization of the products obtained from 1 h treatment of non-activated and mechanically activated pyrites at 713 K under inert atmosphere and characterization of non-activated and mechanically activated pyrites exposed to ambient air for a certain period.