The transformation of octacalcium phosphate (OCP) to hydroxyapatite (HA) was induced by electron beam irradiation in transmission electron microscopy (TEM). The transformation process and crystal structure changes were examined via bright-field images, electron diffraction, high-resolution transmission electron microscopy (HRTEM) and the fast Fourier transformation pattern of HRTEM images. The results indicate a mechanism of solid-state transformation occurring in the OCP crystals. The TEM examinations and crystal structure analysis also indicate the crystallographic orientation of OCP (010)//HA (01¯0) and OCP (001)//HA (001¯), which differs from the previously proposed orientation of HA growth on an OCP apatite layer.