Amorphous calcium phosphate nanoparticles were synthesized by mixing aqueous solutions of Ca(NO3)(2) and (NH4)(2)-HPO4. Then the particles were matured with sodium bis-(2-ethylhexyl) sulfosuccinate (AOT) at various temperatures. Rod-shaped hydroxyapatite (HAp) nanoparticles were formed at maturation temperature >= 40 degrees C. These results indicate that addition of AOT nor during the particles synthesis but during the maturation at more equal critical temperature is a key to form the rod-shaped HAp nanoparticles using AOT.