The　formation　process　of　hydroxyfluorapatite　under　hydrothermal　environment　has　been　studied　in　detail.　The　fluoride　ion　has　been　substituted　for　the　hydroxide　ion　in　the　OH　lattice　position,　which　has　been　characterized　by　X-ray　diffraction　(XRD)　and　FT-IR　spectra.　On　the　basis　of　the　experimental　results　obtained　from　the　transmission　electron　microscopy　(TEM)　and　high-resolution　transmission　electron　microscopy　(HRTEM),　a　detailed　four-step　growth　mechanism　has　been　proposed　for　the　formation　of　the　3D　nanostructure,　which　involves　the　initial　nucleation　(conventional　hydrothermal　crystallization　process),　the　self-assembly　of　nanoparticles　(oriented　attachment),　the　self-assembly　of　thin　nanorods　(oriented　attachment)　and　the　grain　growth　(Ostwald　ripening).　(C)　2009　Elsevier　B.V.　All　rights　reserved.