AbstractHydroxyapatite‐glass‐titanium (HA‐G‐Ti) functionally gradient composite plates bound face‐to‐face by nylon wire were soaked in a simulated body fluid. The gap between the plates was filled with the precipitates after soaking for 6 months, and a tight chemical bond was formed between them. Field emission‐type (FE) scanning electron microscopy (SEM) and transmission electron microscopy observations revealed that the growth rate and morphology of the precipitates differed widely from the outside portion to the inside one on the surface region of the composite plates. In the portion relatively near the outside, the mutually bonded portion, needle‐like precipitates with preferred orientation to the c‐axis were markedly observed. From the results of Fourier transform infrared spectroscopy and energy dispersive x‐ray microanalysis, it was found that the precipitates were Ca‐deficient carbonate apatite with low crystallinity. On the other hand, HA‐G‐Ti composite rods were implanted in the femur of beagle dogs for 3 months. FE‐SEM observations demonstrated that needle‐like precipitates were deposited on both the surfaces of the composite and bone at the portion having a narrow gap, about 0.1–0.2 mm between them prepared deliberately before implantation. It was found that both the morphologies and the composition of the precipitatesin vivowere similar to thosein vitro.It is concluded that the HA particles on the surface of the composite act as nucleation sites for precipitation in physiologic environments, whereas the glass matrix is independent to