SiO₂ source on the hydrothermal synthesis of 1.1-nm tobermorite was studied by using natural zeolite. The mixtures of calcium hydroxide and natural zeolite with the Ca(Si+Al) ratio of 0.8-1.0 were reacted hydrothermally under saturated steam pressure, at 150-230 ℃ for 8-48 hour. In result natural zeolite yielded tobermorite at the initial stage of the reaction. The highly crystalline tobermorite was formed at 0.8 mole and 230 ℃ for 48 hour as optimum conditions of hydrothermal synthesis. In the presence of Al in starting, tobermorite was crystallized more rapidly at 0.8 mole than 1.0, but in the absence of Al, crystallization was faster at 1.0 mole than 0.8. Tobermorite showed anomalous thermal behaviour without any significant change in the basal spacing of a heated sample at 700 ℃ for a week. In particular the presence of aluminum in the starting accelerates the crystallization and stabilization of tobermorite, and that the aluminum ion substitutes for the silicon ion in the tobermorite structure.