Phosphorus-substituted tobermorite was easily synthesized by hydrothermal treatment using phosphorus-doped calcium silicate hydrate as a starting material. The crystalline phase of phosphorus-substituted tobermorite was identified by X-ray diffraction pattern analysis. The diffraction patterns of phosphorus-substituted tobermorite showed good agreement with the powder patterns in tobermorite. The single phase of tobermorite was confirmed. With increasing P/(P+ Si) molar ratio, the lattice constants, i. e., the a and c-axis distances, of phosphorus-substituted tobermorite decreased. A maximum P/(P+ Si) molar ratio of 0.10 was incorporated in the structure of tobermorite. Increasing the P/(P+ Si) molar ratio was confirmed by the adsorption bands of HPO42- and PO43- groups by infrared adsorption measurement. The morphology was observed by scanning electron microscopy. The primary particles were thin plate-like crystals of about 0.1 mu m (thickness) x 1.0 mu m (width). The secondary particles were formed into approximate spheres of about 20 mu m in size by aggregation of the thin plate-like crystals. (C) 2014 The Ceramic Society of Japan. All rights reserved.