Nucleation of hydroxyapatite (HAP) nanocrystals from amorphous calcium phosphate (ACP) nanoparticles in a stoichiometric calcium/phosphate aqueous solution was directly observed by dark field transmission electron microscopy. This observation is a direct evidence of internal arrangement process for the ACP-HAP conversion, while it is against the view of ACP dissolution/reprecipitation for HAP formation in a stoichiometric calcium/phosphate aqueous solution.