Equal Channel Angular Pressing (ECAP) in a fully pearlitic structured steel 65Mn was successfully carried out at 923 K via route C in this study. The severe shear deformation of ECAP was accommodated by periodical bending, periodical shearing and shearing fracture etc in the pearlitic lamellae. The cementite in the pearlite has higher plastic deformation capability. Excessive imperfections may be introduced into the cementite, which supplies additional energy driving for the following spheroidization of cementite in subsequent processing. After five ECAP passes, the fully pearlitic lamellae evolved into a microstructure of ultrafine-grained ferrite matrix uniformly dispersed with finer cementite particles. The ferrite matrix is homogeneous with an average grain size of ~0.3 micrometers. Two possible mechanism for the spheroidization of cementite were proposed：heterogeneous growth of the fractured cementite fragments, and the precipitation of new fine spherical cementite particles through nucleation and growth.