Large-sized hydroxyapatite (HA) whiskers with length of 50-100 mu m and width of 0.5 mu m were synthesized by a facile hydrothermal method. Dilute reaction solutions containing glutamic acid were used in order to achieve low degree of supersaturation with respective to HA precipitation. The supersaturation values at different hydrothermal temperatures were calculated theoretically from ion concentrations with considerations of all association-dissociation balances between various ions. Experimentally, it was found that HA whiskers with smaller sizes were obtained when raising the additive amount of NaOH or glutamic acid. The regulation mechanism of crystal growth along c-axis was proposed according to the degree of supersaturation, the adsorption/desorption behaviors of glutamate occurred on HA's (1 0 0) and (0 0 1) planes, as well as the concentration effect of glutamate's selective adsorption. (C) 2010 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.