The hydroxyapatite (HAp, Ca-5(PO4)(3)OH) nanofibers with a diameter about 2 nm and a length above 400 nm were synthesized by using oleic acid as capping and structure-directing agents under 140 degrees C hydrothermal condition. The structure and morphology of nanofibrillar HAp were characterized by means of X-ray diffraction, transmission electron microscopy, Fourier-transform infrared spectroscopy and thermo-gravimetric and differential thermal analysis, respectively. The growth dynamics and evolution of nanocrystals were investigated by controlled experiment on the synthesis conditions, including the molar ratio of reactants, temperature and time of hydrothermal treatment. The proposed mechanism of the nanofibers formation, growth and self-assembly mediated by organic agents might facilitate the design and synthesis of nanostructural materials with desirable sizes and shapes for consequent use in the related fields. (C) 2014 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.