Abstract: Onset, propagation, formation of shear band and lateral displacement of rock specimen with a material imperfection in the form of a null element at left edge in plane strain compression were modeled numerically by FLAC. The failure criterion was a composite Mohr-Coulomb criterion with tension cut-off. Linear post-peak strain-softening constitutive relation of rock was adopted. It is found that lateral displacement of specimen is non-uniform. Lateral displacement near top and base of specimen is lower than that close to the middle. In the vicinity of top and base of specimen, compressive stress-lateral displacement curve exhibits snap-back behavior (Class II behavior) once peak stress is reached, owing to that the recovery of elastic strain is faster than increase of plastic strain. Generally, onset of strain localization can be identified by lateral displacement-time step curves measured at different locations within specimen. Strictly speaking, onset of strain localization corresponds to the beginning of separation of the curves at the same height. After localization begins, lateral displacements at two edges are asymmetry and change in displacements is non-synchronism. If time steps of transition from symmetry with respect to veridical axis of specimen to non-symmetry are obtained, then onset of strain localization can be identified numerically.