Strain localization and macroscopically mechanical behaviors of rock specimen with and without material imperfection in plane strain compression were modeled numerically by FLAC. The adopted failure criterion was a composite Mohr-Coulomb criterion with tension cut-off and the post-peak constitutive relation of rock was linear strain-softening. For ideal specimen without any material imperfection

the deformation of the specimen is symmetrical with respect to the vertical axis of the specimen. If an imperfection is introduced in the form of a null element at one lateral edge

then the deformation no longer remains symmetrical. Shear strain localization is mainly initiated in the vicinity of the imperfection. For imperfect specimen

the onset of strain localization is earlier than that of ideal specimen. When the imperfection approaches to the middle of left edge of the specimen

multiple shear bands are formed so that the relatively ductile shear failure modes are expected and the post-peak stress-axial deformation curve and stress-lateral strain curve of the specimen tend to be less brittle. If the imperfection is closer to the top and base of the specimen

only a shear band is formed

going through the two lateral edges of the specimen

and the macroscopically mechanical behaviors of the specimen tend to be more brittle at post-peak. Migration or jump of shear band and competition between the two shear bands are observed. For ideal specimen

the peak strength is higher than those of imperfect specimens. When the imperfection moves away from the fixed end (base of the specimen)

the peak strength decreases and finally approaches to a constant.