The observation of scanning electron microscope (SEM) showed that a tibia bone is a kind of bioceramic composite consisting of hydroxyapatite layers and collagen protein matrix. All the hydroxyapatite layers are parallel with the surface of the bone and consist of numerous hydroxyapatite sheets. The observation also showed there is a kind of intercrossed microstructure of the hydroxyapatite sheets. In which the hydroxyapatite sheets in an arbitrary hydroxyapatite layer make a large intercrossed angle with the hydroxyapatite sheets in its adjacent hydroxyapatite layers. The maximum pullout force of the intercrossed microstructure, which is closely related to the fracture toughness of the bone, was investigated and compared with that of the parallel microstructure of the sheets through their representative models. Result indicated that the maximum pullout force of the intercrossed microstructure is markedly larger than that of the parallel microstructure.