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 with different heights and width are conducted. To use the rectangular specimen is convenient for this object. Shear strain distribution localized with compression is also successfully observed by taking digital photograph from two side of rectangular specimens and image analysis of their digital photograph. It is found that the bifurcation phenomena, e.g. formation and progress of various shear planes, failure with buckling, existence of a bifurcation point and following instability on load - displacement curves, can be clearly observed in the present tests.