The mixture of quartz powder and slaked lime with plenty water was oscillated in an autoclave and treated hydrothermally under the pressure of 10 kg/$ extrm{cm}^2$ for 8 hours. The main mineral synthesized was confirmed 11$AA$ tobermorite by the method of X-ray diffraction and selected area electron diffraction. Tobermorite was heat-treated at 40$0^{circ}C$, 80$0^{circ}C$ and 100$0^{circ}C$ for investigation of transformation of morphology and structure. Electron micrographs showed the thin platy structure of synthetic tobermorite with a little of crumpled foil or fibrous semi-crystalline calcium silicate hydrates. No difference in structure was appeared under the temperature of 80$0^{circ}C$ but tobermorite converted gradually into wollastonite at 800$^{circ}$~85$0^{circ}C$. On heating, moulded material from tobermorite hardly shrinks under 80$0^{circ}C$.