Bone is a kind of biomaterial in nature. It behaves favorable strength, stiffness and fracture toughness which are closely related to its fine microstructure. SEM observation on a shankbone shows that the bone is a kind of natural bioceramic composite consisted of hydroxyapatite layers and collagen matrix. The observation also shows that the hydroxyapatite layers consist of many hydroxyapatite sheets and are arranged in a parallel distribution. The fracture toughness of the bone is analyzed based on the representative model of the microstructure of the bone and the idea of maximum pullout force. The analytical result shows that the long and thin shape as well as the parallel distribution of the hydroxyapatite sheets improves the maximum pullout force of the sheets and the fracture toughness of the bone.