Transformation of octacalcium phosphate (OCP) with incorporated succinate ion to hydroxyapatite (HAp) under hydrothermal conditions was investigated herein. Plate-shaped OCP with incorporated succinate ion was transformed to HAp composed of laminated thin plate-shaped crystals, confirmed by morphological observations and X-ray diffraction analysis. The organically modified OCP is useful as the precursor for HAp crystals with unique nanostructures.