Bone toughening and fracture mechanics are an important area of study for medical scientists seeking to predict and prevent fracture risk, and for engineers interested in designing novel, biologically-inspired materials. This paper reports on the effect of internal bone microstructure upon microcrack propagation trajectory in bovine osteonal cortical bone. A two-dimensional micromechanical fibre-reinforced composite materials model was generated using the finite element method. Interstitial tissue was modeled as a matrix, osteons as fibres, and the ‘cement line’ as an interface between osteons and interstitial tissue. Fracture tests on compact tension samples of bovine femur were performed and compared to modeling predictions. Micrographs of fracture surfaces were obtained using scanning electron microscopy. Results show that cortical bone microcrack propagation is greatly influenced by osteonal density, suggesting bone resistance to fracture can be predicted, at least in part, by quantification of osteonal density.