Hydroxyapatite (HA) crystals were synthesized from calcium acetate monohydrate and phosphoric acid using hydrothermal method. The various interactions of reactant concentrations (1 – 3 Molar), surfactant {aminotris (methylene phosphonic acid) [N(CH₂PO₃H₂)₃] (ATMP)} concentrations (0 - 100 ppm), and hydrothermal times (6 – 24 hours) were investigated and their effects on the mean diameter of the HA crystals were obtained using the Box-Behnken experimental statistical design. Results have shown that surfactant has no effect on the mean diameter of the crystals. On the other hand, the results revealed that time and reactant concentration are major parameters in changing the particle size of hydroxyapatite crystals. Without addition of the surfactant, well elongated crystals with high degree of crystallinity were synthesized. With addition of the surfactant, the agglomeration of HA particles as well as hardness of HA pressed discs are significantly increased. HA particles were ranged from 2.7 µm to 7.2 µm as crystal aggregates whereas the obtained crystallite sizes ranged from 17.3 nm to 30.3 nm.