Nanorod and needle-like hydroxyapatite (HAp) crystals were successfully synthesized at 85 degrees C through a simple and mild pathway. Single-phase HAp powder with high crystallinity was obtained without thermal treatment. The effect of urea concentration on the crystal component was investigated. X-ray diffraction (XRD) results indicated that pH adjustment played a key role in controlling the composition of final products. The fast adjustment on pH by NH3 center dot H2O resulted in formation of single-phase HAp with nanorod and needle-like shapes while the slow adjustment by urea led to a mixture of octacalcium phosphate (OCP) and HAp with ribbon-like morphology. Ribbon-like OCP crystal was the intermediate for the transformation from monetite (DCPA) to HAp. (C) 2009 Elsevier B.V. All rights reserved.