Strain localization, the phenomenon of large shear deformation within thin zones of intensive shearing, commonly occurs both in-situ and in the laboratory tests on soils specimens. The intriguing mechanism of strain localization and how it will affect the general behavior of soil specimens have been investigated by many researchers. Some of the efforts have focused on finding the links between material properties (void space, fabric tensor) and mechanical behavior (stress, strain, volumetric strain). In the last ten years, several extensive studies have been conducted at Georgia Tech to investigate the mechanism of strain localization and link the microstructural properties with the engineering behavior of Ottawa sands. These studies have included 2-D and 3-D characterization of soil microstructures under either triaxial or biaxial shearing conditions. To extend and complement these previous studies, the current study focuses particularly on 3-D reconstruction, analysis and modeling of specimens of Ottawa sand subject to triaxial or biaxial loading. The 3-D microstructure of biaxial specimens was reconstructed using an optical microscopy based montage and serial sectioning technique. Based on the reconstructed 3-D digital volumes, a series of 2-D and 3-D characterizations and analyses, including local void ratio distributions, extent of shear bands, influence of soil fabrics and packing signature effects, were conducted. In addition to the image analysis based reconstruction and characterization, the 3-D discrete element method (DEM) code, PFC3D, was used to explore both biaxial and triaxial shear related soil behaviors at the global and particulate scale. Void ratio distributions, coordination numbers, particle rotations and displacements, contact normal distributions and normal contact forces as well as global stress and strain responses were investigated and analyzed to help understand the mechanism of strain localization. The microstructures of the numerical specimens were also characterized in the same way as the physical specimens and similar strain localization patterns were identified. Combined with the previous related studies, the current study provides new insights into the strain localization phenomenon of Ottawa sands subject to triaxial and biaxial loading. In addition, the reconstructed digital specimens were subject to a series of dissection studies which revealed exciting new insights into "microstructure signatures" which exist at both meso and micro scales within the real and simulated specimens.