The atomic structure of {10 (1) over bar0} interfaces in sintered hydroxyapatite (HAp) was characterized using high-resolution electron microscopy (HRTEM). When subjected to electron beam radiation parallel to the [0001] zone axis, a characteristic damage of HAp occurred. The damage was identified by hexagonally shaped regions surrounded with {10 (1) over bar0} planes, in which the crystalline phase was thinned and amorphized. HRTEM study at the crystalline-amorphous interfaces revealed that the HAp crystal structure was terminated by a plane crossing the hydroxyl columns on which Ca (Ca2 site) and PO4 tetrahedra were placed. The grain boundaries parallel to the {10 (1) over bar0} planes were also examined and the structure was identical to that observed in the crystalline-amorphous interface. The interface structure observed in the crystalline-amorphous interfaces and grain boundaries is probably a stable atomic arrangement of HAp {10 (1) over bar0} surfaces with a low surface energy.