Partially saturated soil shows more brittle mode of failure with clearer shear band than fully saturated soil and air-dried soil because of loss of suction caused by shearing or water-infiltration. The failure of partially saturated soil is closely related to strain localization which is considered as one of major failure mechanism for granular materials. It is, therefore, quite important to investigate the grain-scale shear behavior in strain localized area during shear deformation. In the present study, the mechanism of brittle failure of partially saturated soil is investigated by comparing the behavior of partially saturated soil with that of fully saturated soil under triaxial compression. Development of shear band for fully and partially saturated sand specimens are observed by microfocus x-ray tomography during triaxial compression tests. Local deformation focusing on strain localized area is observed with higher resolution. Digital image correlation with sub-voxel refinement is performed to calculate the displacement field and shear strain filed in three-dimensions based on the CT images with two kinds of resolution, by which discuss the difference of shear deformation between partially saturated sand and fully saturated sand.