We investigated the crystal growth of hydroxyapatite (HAp) from ¡-phase (¡-TCP) and ¢-phase (¢-TCP) tricalcium phosphates showing different solubility, under hydrothermal conditions. Samples of ¡-TCP and ¢-TCP were treated up to 72 h under hydrothermal conditions at 120°C. The ¡-TCP sample was completely transformed into HAp crystals within 1 h after the powder was exposed to hydrothermal conditions. In contrast, the ¢-TCP sample required more than 72 h to achieve complete transformation into HAp crystals. This difference in transformation behavior affected the morphology of the formed HAp crystals, which were observed to form plate-and rod-shaped crystals from ¡-and ¢-TCP, respectively. The high dissolution rate of ¡-TCP led to fine, plate-shaped crystals whose morphology was maintained up to 72 h once they formed. In comparison, the low dissolution rate of ¢-TCP allowed production of large, well-crystallized HAp crystals through the hydrothermal treatment conditions, even though the chemical composition of the reactant samples were the same. The morphology of the HAp crystals formed as a result of the hydrothermal conditions was controlled by selecting starting materials whose solubility affects an increase in the degree of supersaturation in the solution.