Bone is the principal structural component of a skeleton with a hierarchical structure in multiple length scales, which shows unique mechanical, biological, and chemical characters. Bone fractures are affected by factors such as changes in the material's microstructure and properties and the accumulation of microcracks. Therefore, the study of bone microstructure effects on crack propagation at the microscale is of particular importance. In this study, two-dimensional finite element models of human cortical bone tissue and bovine cortical bone tissue were created as three-phase composite and four-phase composite under tensile load. Then, crack propagation analysis inside the bone representative volume element was performed using the extended finite element method inside Abaqus software, and local sensitivity analysis was performed. The resulting stress-strain behavior, which is quite different in the two studied models, confirms the critical role of bone tissue microstructure in the failure. The results emphasize the importance of the interphase, i.e. cement lines in the four-phase composite model. The presence of this interphase locally affects the crack propagation path and behavior at the macro scale. Two models with different porosity percentages and different porosity positions are studied. It was found that cavities affect the crack propagation path, and increasing the porosity percentage reduces the maximum stress required to start crack propagation. By looking at two models with different ratios of osteon and cement line, it was found that the percentage effect of these two components is significant in bone fracture, so bone mineral density is one of the crucial parameters in bone fractures. By performing sensitivity analysis, it was found that the elastic modulus of the cement line has a more significant effect on the final strength, and the porosity percentage has the most significant impact on the final strength. The experimental results obtained from the literature in this field were used to develop and validate numerical models. Keywords: Cortical Bone Tissue, Microstructure, Extended Finite Element Method, Fracture, Sensitivity Analysis, Cement Line.