Accurate knowledge of bone fracture healing process is of clinical and theoretical importance in bone repair and regeneration, and biomineralization. It is well known that the histological healing occurs via the formation of hematoma, fibrocartilage, bony callus and bone modeling/remodeling. However, the detailed process from fracture to healing at the microstructural level remains unclear. In the present study, an evolutionary model of external callus is proposed, in which five representative stages are presented in terms of the organization of collagen and minerals during the formation of bony callus. The first stage is the formation of loose, disordered collagen fibrils, which is followed by mineralization on some of these individual microfibrils. Then the matrix is characterized by the fusion of mineralized individual fibrils into bundles. In the third stage, the absorption of disordered matrix occurs. This is gradually replaced by ordered collagen in stage four. Finally, completely ordered mineralized tissue is formed. The proper sequence of the process plays an important role in deciding the success of healing. In addition to the common mineral phase of hydroxyapatite (HA), dicalcium phosphate dihydrate (DCPD) phase was also found in early stage of healing, especially in rapid healing (children's callus). It Vanished in the following process of healing. The deposition of DCPD is supposed to be brought about by some non-collagenous protein. (C) 2000 Elsevier Science S.A. All rights reserved.