Most structural materials existing in nature take the form of composite. After centuries’ evolution, these materials gain highly optimized microstructures and performances. In this work, a kind of natural biomaterial, shinbone, is observed with a scanning electron microscope (SEM). The observation result shows that the bone is a bioceramic composite consisting of hydroxyapatite layers and collagen matrix. The observation also shows that the hydroxyapatite layers are composed of hydroxyapatite sheets. The hydroxyapatite sheets are of thin and long shape and parallel distribution along the orientation of the maximum main stress of the bone. The shape and distribution of the hydroxyapatite sheets may endow the bone with favorable fracture toughness, which is analyzed and illuminated based a representational model of the hydroxyapatite sheets and the idea of maximum pullout force.