Nanocomposites of hydroxyapatite (HAp) and collagen (Col) were prepared by a biomimetic coprecipitation method under controlled pH and temperature. Transmission electron micrographs showed that the c-axis of HAp nanocrystals aligned along collagen by a self- rganization mechanism; a very similar alignment to bone's structure was found in the sample prepared at nearbiological conditions, i.e. pH8–9 and 40'C, which consisted of fibers with length of 20μm. From the material scientific point of view, therefore, osteoblasts may control the chemical condition in a small area around the cells to pH 8–9 as well as supply raw materials of bone, hydroxyapaite and collagen, at the initial state of osteogenesis. The biocompatibility of the composites was examined by animal tests using beagles. The composites were absorbed and new bone was formed as rapidly as a bone remodeling process. The HAp/Col composites prepared by biomimetic process were expected to be applicable as filler materials changing to new bone instead of autograft.