Qualitative and quantitative thermal behavior of carbonate ions incorporated into hydroxyapatite with various Sr-Ca compositions was investigated. Desorption of CO_2 from apatite by heating and the amount of CO_2 that evolved from the apatite were strongly influenced by the Sr content. Sr-60%-substituted apatite (designated as Sr60) showed the lowest peak temperature and the most desorbed amount of CO_2 of all the samples Sr0, Sr20, Sr40, Sr60, Sr80 and Sr100. The dependence of the peak temperature on Sr content was similar to that of the thermal stability of the apatite on Sr content, suggesting that the apatite with lower thermal stability was able to incorporate more carbonate ions into its structure.