Effects of added alpha-tricalcium phosphate (alpha-TCP) and beta-TCP were investigated to shed light on the setting reaction of apatite cement (AC) consisting of tetracalcium phosphate (TTCP) and dicalcium phosphate anhydrous8 (DCPA). Added beta-TCP showed no reactivity, and thus resulted in extended setting time and decreased mechanical strength. In contrast, alpha-TCP dissolved to supply calcium and phosphate ions after initial apatite crystal formation by the reaction of TTCP and DCPA. Although setting time was delayed because alpha-TCP was involved only in the latter reaction of apatite cement, larger apatite crystals were formed due to its addition. As a result of larger apatite crystal formation, the mechanical strength of alpha-TCP-added apatite cement increased by approximately 30%, as compared to alpha-TCP-free apatite cement.