Hydroxyapatite (Ca 10 (PO 4) 6 (OH) 2 ; HAp) is widely used as a biomaterial due to its high biocompatibility. However, biological apatite present in bone and teeth contains various ions in the HAp crystal structure. Thus, biological apatite has many strains and defects, which may impart high osteoconductivity to apatite. To clarify the effects of the strain and/or defects in the HAp crystal structure on the bioactivity, nitrogen-doped (N-doped) HAp ceramics were fabricated by heating pure HAp ceramics. This N-doped method is well-known as a technique to modify the chemical structure at the surface. Some properties of the N-doped HAp ceramics were examined for optimization of the heating temperature. N-doped HAp ceramics fabricated by heating at 850°C in an NH 3 atmosphere had N 2 O molecules in the crystal structure. Therefore, the N-doped HAp ceramics fabricated by the N-doping method have a rare moleculesubstitution structure. In conclusion, we have developed the N-doped HAp ceramics which have a rare molecule-substitution structure.