In order to clarify important factors which govern the tobermorite formation in hydrothermal reaction, the effects of quartz particle size and water content were investigated for simplifi ed model systems by in-situ XRD and Si-NMR. Although the reaction time was largely dependent on the quartz particle size, the average Ca/Si in non-crystalline C-S-H at the time that either tobermorite or semi-crystalline C-S-H start to be formed (T0) was nearly identical for all samples. The importance of molar ratio Ca/Si as well as the silicate chain length in non-crystalline C-S-H was suggested. Tobermorite can be synthesized even in the systems with large w/s and fi ne quartz. It was suggested that lower calcium and/or silicate ion concentration in the solution phase is important for tobermorite formation.