Background: Calcium phosphate cements (CPCs) show several advantages over other materials, used for bone repair. For example, they are injectable, easily shapeable and remained localized. Therefore, they fill effectively bone defects with an irregular shape. Furthermore, CPCs are very bone compatible and also osteoconductive. Objectives: This work aimed to investigate the effect of particle size on the mechanical and setting properties of α-tricalcium phosphate (α-TCP) based cements. The rate of conversion of reactants to nano-hydroxyapatite (nHA) in the medium of human blood plasma is also studied. Materials and Methods:In this study, we prepared CPCs consisting of α-TCP (61%), dicalcium phosphate (DCP) (26%), calcium carbonate (CaCO 3 ), hydroxyapatite (HA) (3%) as powder phase, in a solution of 3 wt% NaH 2 PO 4 as liquid phase. In the next step, three different cements with the same formulation but different α-TCP particle sizes (4 μm, 10 μm, 22 μm) were prepared. Finally, we evaluated the setting time, compressive strength and the rate of conversion of reactants to apatite phase in blood plasma. Results: Based on the results, the initial setting time decreased from 30 minutes for CPC with α-TCP particle size of 22 μm to 15 minutes for the cement with α-TCP particle size of 4 μm. Also, the cement prepared with the least α-TCP particle size exhibited the maximum compressive strength after setting. The results revealed that reduction of α-TCP particle size, the main component of the CPC, favors conversion of cement constituents to needle-like nano-apatite crystals when soaking in human blood plasma, and this leads to increment of mechanical strength. Conclusions: In α-TCP based CPCs, reduction of α-TCP particle size favors the conversion of the cement constituents to nano-apatite crystals (when soaking in human blood plasma), which leads to reduction of setting time and increase in mechanical strength of CPCs. Implication for health policy/practice/research/medical education:Calcium phosphate cements (CPCs) show several advantages over other materials used for bone repair. For example, they are injectable, easily shapeable, and remained localized. Therefore, they fill effectively bone defects with an irregular shape. In this study, the effect of reactants particle size on some key properties of CPCs was investigated, which helps improving the quality of these cements.