Hydroxyapatite fiber (fHA) was hydrothermally synthesized using propionamide (PA) as a pH adjusting agent. The concentrations of PA and calcium ion (Ca), as well as synthetic temperature and time, were regulated and investigated in detail to explore the formation mechanism of fHA. The results demonstrated that the morphology of fHA was susceptible to the concentrations of PA and Ca, while the composition and crystallinity were not. The morphology of fHA changed from long fibers to flowers and thorn-like bundles with increasing concentrations of PA and Ca, respectively. Higher temperature was shown to contribute to the generation of highly crystalline fHA. By further combining the theories of oriented attachment and Ostwald ripening with spiral growth mechanism, it was proposed that dicalcium phosphate (DCPA) sheets were first formed via the self-assembly of nanoplatelets derived from nucleation. The sheets were then converted into the final fHA through spiral growth. (C) 2013 Published by Elsevier B.V.