The microstructures of the nanocrystalline surface layer of a quenched and high temperature tempered 0. 4 C- 1 Cr steel induced by high-power surface processing (HPSP) technique were characterized by scanning electron microscopy and transmission electron microscopy. The results indicate that a nanocrystalline layer was fabricated on the surface of the steel by using HPSP treatment. The mean grain size in the surface layer is about 11 nm. The nanocrystallization of cementite is prior to that of the matrix phase, ferrite.