We investigated the crystal growth of hydroxyapatite (HAp) from α-phase (α-TCP) and β-phase (β-TCP) tricalcium phosphates showing different solubility, under hydrothermal conditions. Samples of α-TCP and β-TCP were treated up to 72 h under hydrothermal conditions at 120°C. The α-TCP sample was completely transformed into HAp crystals within 1 h after the powder was exposed to hydrothermal conditions. In contrast, the β-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 α- and β-TCP, respectively. The high dissolution rate of α-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 β-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.