The paper discusses the mechanisms of formation of nanostructure, chemical and phase composition, and dispersity of the particles of metal powders obtained in the process of mechanochemical activation and of mechanical alloying. It has been shown that the processes of plastic deformation and disintegration of the particles are realized via macrodislocations, which are the linear defects of nanostructure. The moving macrodislocations create around themselves the clouds of impurity atoms, the transfer of which determines the mechanism of strong supersaturation with impurities and the structural-phase composition of the powders obtained. The mechanisms of the mechanical alloying of iron with carbon have been studied. A model has been proposed for the nucleation and growth of cementite via the formation of the nonequilibrium interstitial phases. Carbon segregations have been investigated, the distribution and density of which are mainly non-uniform and are fully determined by the interstitial phase structures. Within the frames of the model proposed the existence of an amorphous iron phase in a special "cluster-isolated" state has been predicted. (C) 2004 Kluwer Academic Publishers.