<p>The failure of rocks is frequently driven by stress-induced tensile failure. Tensile stress plays an important role during the formation and deformation of rocks. The microstructural characteristics of rocks pose a crucial impact on their deformation and fracture. Therefore, it is of great significance to investigate the effect of microstructural characteristics on the mechanical property and fracture mechanism of heterogeneous rocks for engineering applications. This paper investigated the effects of microstructural characteristics on the mechanical properties and fracture mechanisms of heterogeneous rocks under tensile loading using image-based discrete element models (DEMs). Four rock types—coarse-grained granite, fine-grained granite, marble, and basalt—were modeled, incorporating realistic mineral size, shape, and distribution. Brazilian tests were conducted to analyze shear and tensile crack distribution in tensile and compressive zones. A 2D digital image correlation (2D-DIC) technique was applied to differentiate intergranular and transgranular cracks during failure. Results show that rock microstructure significantly affects splitting failure. As mineral complexity increases, the height of tensile zone in Brazilian disks declines. In the tensile zone, tensile cracks dominate, though some shear cracks occur. The tensile-to-shear crack ratio in the compressive zone decreases with increasing mineral distribution complexity. Upon complete fracture, transgranular cracks accounted for 58%, 63%, and 53% in coarse-grained granite, fine-grained granite, and basalt, respectively, with a higher proportion along the loading diameter than intergranular cracks. These findings offer a valuable grain-scale tool for analyzing rock failure in engineering applications such as hydraulic fracturing, tunneling and slope excavation.</p>