Octacalcium phosphate (OCP), a layered calcium phosphate compound, has attracted attention in areas such as biomaterial and pharmacy, as well as the environmental industry, because of its excellent biocompatibility and low environmental load. However, little is known about the effect of alkaline metal ions on OCP formation, even though the Na ion is always present as a pH neutralizing or ionic strength-adjusting agent. Therefore, in this study, we investigated the role of the Na ion in OCP formation from dicalcium phosphate dihydrate through hydrolysis by using solutions with various Na concentrations. When the Na concentration in the treatment solutions increased, the formation of hydroxyapatite as a residual material was inhibited and the purity of OCP was increased. Furthermore, at higher concentrations, OCP crystals evolved layer structures because Na affects the P5 PO4 in the OCP lattice and enhances the HPO4-OH layer structure, which significantly contributes to OCP crystallinity and crystal structure. Thermal stability measurements indicate that the thermal stability of OCP decreases with increasing Na content. This study suggests that Na incorporation into OCP crystals can induce OCP formation but simultaneously decreases the OCP stability.