Hydrothermal formation reaction of tobermorite has been investigated by in situ X-ray diffraction (XRD) using high-energy X-rays from a synchrotron radiation source in combination with a newly developed autoclave cell and a photon-counting pixel array detector. XRD measurements were conducted in a temperature range of 100°–190°C throughout 7.5 h of reaction time with a time interval of 4.25 min under a saturated steam pressure. Two samples using different types of cements were compared: high early strength Portland cement and ordinary Portland cement. In both cases, noncrystalline calcium silicate hydrate (C–S–H), hydroxylellestadite (HE), and katoite were clearly observed as intermediate materials. A possible mechanism that the consumption of noncrystalline C–S–H and HE as precursors of tobermorite occurs sequentially rather than simultaneously has been suggested. Four tobermorite peaks corresponding to different reflections started to be detected at the same timing, suggesting tobermorite is formed without any preference for a particular crystal axis at the beginning. A decrease in the Bragg spacing of the basal reflection during the reaction was observed, implying the increase of periodicity along the c-axis.