The structural transformation, microstructural changes, and nitrogen sorption of crystalline Si induced by high-energy ball milling in ammonia have been investigated. It is found that significant refinement of particles and crystallites occurs at the early stage of milling (within similar to 10 h of milling), after which the average particle size and crystallite size decrease gradually and approach a lower limit of similar to 100 and 6.0 nm, respectively. It is proposed that refinement of particle sizes is dominated by the fragmentation process, whereas refinement of crystallite sizes is dictated by plastic deformation, Substantial amorphization does not occur until most of the crystallites have reached the average size of 6.5-6.0 nm, The crystallite-refinement-induced amorphization has been identified as the major mechanism for the nanocrystalline to-amorphous transformation. A substantial amount of nitrogen is sorbed by Si powders and primarily dissolved in the amorphous Si phase.