As a typical biological material, bone possesses high fracture strength and fracture toughness, which are closely related to its exquisite microstructure. SEM observation of a cannon bone shows that the bone is a kind of layered bioceramic composite consisting of hydroxyapatite sheets and collagen matrix. The hydroxyapatite sheets are of long and thin shape, distributing in parallel. The fracture toughness of the bone is analyzed with the representative model of the hydroxyapatite sheets and the concept of maximum pullout energy. It is shown that the lathy shape as well as the parallel distribution of the hydroxyapatite sheets increases the pullout energy and endows the bone with high fracture toughness.