In order to understand further the mechanism of bone fracture repair, and thus to innovate better operative treatment for bone fracture and to design new implant materials for bone repair, microstructures of external periosteal callus (EPC) of repaired femoral fracture in both children and adults were investigated by using a scanning electron microscope, transmission electron microscopy and an X-ray microdiffractometer, The repair time after the fractures in children and adults is on average 155 and 370 days, respectively, Collagen fibrils making up children's EPC (CEPC) are underdeveloped and insufficiently mineralized by hydroxyapatite (HA), while those from adults' EPC (AEPC) are similar to normal bone, A lot of particles loaded by brushite (DCPD) minerals were found among the collagen fibrils of CEPC, The main mineral phases in CEPC consist of DCPD and HA, while only HA exists in AEPC, Deposition of DCPD minerals could have compensated for the insufficient mineralization of the collagen fibrils of CEPC, thereby making fractured bone repair more rapidly in children than in adults. (C) Academic Press