Bone is an anisotropic structure which can be compared to a composite material. Discontinuities within its microstructure may provide stress concentration sites for crack initiation, but act as a barrier to its propagation. This study looks specifically at the relationship between crack length and propagation in compact bone. Beam‐shaped bone samples from sheep radii were prepared and stained with fluorochrome dyes and tested in cyclic fatigue under four‐point bending in an INSTRON 1341 servo‐hydraulic fatigue‐testing machine. Samples were tested at a frequency of 30 Hz and stress range of 100 MPa under load control. Specimens were sectioned transversely using a diamond saw, slides prepared and examined using epifluorescence microscopy. Cracks in transverse sections were classified in terms of their location relative to cement lines surrounding secondary osteons. Mean crack length, crack numerical density and crack surface density were examined. Short microcracks (100 µm or less) were stopped at the cement lines surrounding osteons, microcracks of intermediate length (100–300 µm) were deflected as they hit the cement line, and microcracks that were able to penetrate through cement lines were longer (> 400 µm). These data show that bone microstructure allows the initiation of microcracks but acts as a barrier to crack propagation.