Abstract: A submicrometer grained 45 steel was prepared by equal channel angular pressing (ECAP) technology at 500 ℃. The behavior of cementite dissolution and spheroidization during ECAP process was studied by optical microscopy and transmission electron microscopy. The results show that the deformation modes of lamellae cementite included bending, tensing, kinking, shearing and shearing fracture during ECAP process. Cementite attenuation, shearing, refinement or a large number of defects on the surface promoted the cementite dissolution due to thermodynamical unstability. The cementite spheroidization was due to the different curvature in the outside of sheared and refined cementite.