Studies on changes in the morphology and structure of nanocrystalline ZnFe2O4, alpha-Fe2O3 and gamma-Al2O3 have shown that at first stages of mechanical activation the activated crystals increase in size. The growth of crystals may be accompanied by change in physical properties of the activated substance. In the case of mechanical activation of aluminium oxide, this is a phase transition. The formation of structural defects during mechanical activation of the substances under study proceeds only after the crystals reach a definite size (of the order of 20 Mm).