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.