Scanning electron microscope (SEM) observation shows that fibula bone is a kind of bioceramic composite consisting of hydroxyapatite layers and protein matters. The hydroxyapatite layers are further composed of hydroxyapatite sheets. The observation also shows that the hydroxyapatite sheets possess quite large volume fraction and also have very long and thin fiber shape. The mechanism of the large volume fraction of the hydroxyapatite sheets to ensure the larger elastic modulus of the bone was investigated based on the model of the bone composite and the theory of the composite mechanics. The investigated result reveals that the large volume fraction of the hydroxyapatite sheets endows the bone with large elastic modulus.