The eXtended Finite Element Method (XFEM) was developed to model both stationary crack problems and crack-growth simulation. It is classified as a singularity-based method or a phantom-node method. In this chapter, XFEM was applied to study the crack growth between two osteons in the cortical bone microstructure. The crack front was created by MESH200, independent of the base meshing. Computational results show that crack growth splits the elements at the crack front without the new meshing. Moreover, the stress intensity factor K of the crack front keeps path independence during the crack growth. A significant drawback of XFEM, however, is that it demands regular meshing at the crack front, which does not exist in most of the practical problems.