The oxidation behaviors of non-activated and mechanically activated galenas were investigated by thermogravimetry method (TG) in flowing highly pure oxygen atmosphere at the heating rate of 10 K min(-1). It is found that the mass increase between 400 and 850 K in the TG curves rises with the increase of grinding time of galena. The difference in oxidation reactivity of non-activated and mechanically activated galenas was also discussed. The specific granulometric surface area (S-G) and the structural disorder of mechanically activated galenas were analyzed by X-ray diffraction laser particle size analyzer and X-ray powder diffraction analysis (XRD), respectively. The results show that the specific granulometric surface area (S-G) of mechanically activated galenas almost remains constant after grinding for a certain period, and lattice distortions (6) increase, but the crystallite sizes (D) decrease with the increase of the grinding time. All the results imply that the mass increase between 400 and 850 K in the TG curves for mechanically activated galenas is mainly caused by the increase of lattice distortions (a) and the decrease of the crystallite sizes (D) with increasing the grinding time.