β-tricalcium phosphate (β-TCP) is usually prepared by burning a mixture of dicalcium hydrogenphosphate anhydride (DCPA) and calcium carbonate (CaCO₃) above 1100°C. However, hydrothermal synthesis may be a more effective process for energy-saving. In addition, amorphous calcium phosphate (ACP) is very useful as a raw material for the morphological control of crystalline calcium phosphates. The present work investigated about the hydrothermal synthesis of β-TCP from ACP. ACP (Ca/P atomic ratio of 1.25-1.55) as starting material was obtained by a hydrolysis reaction of a calcium dihydrogenphosphate monohydrate (MCP) solution. Hydrothermal synthesis of β-TCP occurred under the following conditions: liquid/solid weight ratio, 30; carboxylic acid (formic acid, acetic acid, propionic acid, butyric acid and valeric acid) concentration, 0-1.5mol·dm^-3; temperature, 160-260°C; time, 0-72h. ACP crystallized into DCPA in hydrothermal conditions at 160°C, and was transformed into two phases, β-TCP and DCPA, above 200°C. Single-phase β-TCP formed at 220°C. Hydrothermal synthesis of β-TCP occurred under the following optimum conditions: formic acid solution, 1.0mol·dm^-3; temperature, 220°C; time, 3h. Ca/P atomic ratio of the hydrothermal synthesized β-TCP was 1.48±0.01 and the product was quantified as Ca_2.98(HPO₄)_0.04(PO₄)_1.96.