In the present study, in order to grasp deformation and failure behaviors of clay under 3-D condition as well as large deformation and strain localization, a series of triaxial compression tests using rectangular clay specimens are performed. Various patterns of strain localization are successfully observed by an image analysis. 3-D bifurcation phenomena, e.g. formation and progress of various strain localization patterns, failure with buckling, unstable behavior in the stress-strain relations, can be also observed in the preset tests. Then, a 3-D finite element simulation using an elasto-viscoplastic model for a saturated clay is carried out. The simulated results can well explain the observed strain localization patterns of rectanguler clay sepcimens.