The mechanism of nanocrystallization in steels by ball milling was studied. The importance of severe plastic deformation at high strain rate rather than repeated cold-welding was recognized. This idea was confirmed by the nanocrystallization in other deformation methods such as a ball drop, a particle impact and shot peening deformations those can apply similar severe plastic deformation to specimens with that of ball milling. The nanocrystalline structure produced by these methods has essentially the same characteristics; sharp boundary with work-hardened region, dissolution of cementite, substantial high hardness (about 10 GPa) and without recrystallization and slow grain growth by annealing. It was proposed that a large strain (larger than 7.3 in true strain) and high strain rate (above about 10⁴/s) are necessary deformation conditions to produce nanocrystalline structure in steels. It was suggested that shot peening is the most practical technique to produce nanocrystalline surface layer.