Crystalline layer-structured octacalcium phosphate (Ca8(HPO4)2(PO4)4 5H2O, OCP) and brushite (CaHPO4 2H2O, DCPD) were mechanochemically ground. The crystalline XRD pattern of OCP was weakened with grinding and reached almost amorphously after 72 h. However, the amorphously ground OCP possessed IR spectroscopic and thermogravimetric characteristics of OCP. On the other hand, DCPD was dehydrated to monetite (CaHPO4) by grinding above 4 h. As-prepared OCP particles consisting of aggregates of fine primary thin blade crystals changed to amorphous-like sub-micron spherical particles by grinding. DCPD platy crystals changed to aggregates of very fine primary particles.