First-principles calculations are performed to investigate atomic and electronic structures of Sr2+ ions substituting for Ca2+ in octacalcium phosphate (OCP). The defect formation energies are evaluated from total energies of supercells and ionic chemical potentials of Sr2+ and Ca2+ determined under the chemical equilibrium with aqueous solution saturated with hydroxyapatite (HAp). The defect formation energy depends on the solution pH and the substitutional Ca sites in OCP, and the estimated equilibrium concentrations of Sr2+ in OCP and HAp are in reasonable agreement with previous experimental results obtained in physiological conditions. It is also found that Sr2+ ions can be more favorably substituted in OCP than in HAp. It is thought, therefore, that Sr2+ plays its role to promote bone formation by being incorporated into the metastable OCP phase occurring during HAp nucleation.