We show that acetic acid enhances the HPO₄–OH layer structure of octacalcium phosphate (OCP) and hypothesize that the presence of acetic acid is essential for obtaining large-sized highly crystalline OCP with relatively high-temperature (∼65°C) aqueous-solution synthesis methods. The OCP crystal consists of three types of layer structures (apatite-like, transition, and HPO₄–OH). X-ray diffraction (XRD) analysis indicates that increasing acetic acid concentration leads to the development of their greater structure. Infrared (IR) and nuclear magnetic resonance (NMR) spectroscopic measurements indicated development of the highly HPO₄–OH layer-structure. The enhanced layer structure development caused by acetic acid is in contrast to the generally observed decrease in HPO₄–OH layer structure in the presence of –COOH-containing molecules such as citrate and succinate.