The critical bifurcation orientation and its corresponding hardening modulus for rock-like geomaterials are derived considering the effect of stiffness degradation and volumetric dilatancy under the assumption of isotropic damage. The dependency of the localized orientation on damage degree and initial Poisson’s ratio of rock is examined and the bifurcation behavior of uniaxial tension sample under plane-stress condition is compared with that under plane-strain condition. It is shown that the localization orientation angle intimately depends on both initial Poisson’s ratio and damage degree for the rock sample under uniaxial tension condition. As the initial Poisson’s ratio or damage degree increases

the orientation angle of the plane on which localization tends to be initiated gets to increase. At the same time

the localization orientation angle of rock sample under plane-stress condition is lower than that under plane-strain condition.