The hydrothermal hot-pressing method is a technique to produce ceramic compacts by solidification of inorganic powders under hydrothermal conditions.In this method, a powder with a small amount of water is continuously compressed under hydrothermal conditions from the outside of the autoclave at higher pressure than vapor pressure in the autoclave.The compression accelerates densification of the powder and the hydrothermal treatment forms bonding between the particles.Thus, a compact with high mechanical strength can be produced at low temperatures around 200℃ by this technique.The densification process by the hydrothermal hot-pressing technique is considered to be similar to that of sintering by hot-pressing with the existence of liquid phase.