 This paper summarizes specific features of supercritical hydrothermal synthesis of metal-oxide particles. Supercritical water allows control of the crystal phase, morphology, and particle size since the solvent's properties, such as density of water, can be varied with temperature and pressure, both of which can affect the supersaturation and nucleation. In this review, we described the advantages of fine particle formation using supercritical water and describe which future tasks need to be solved. This article was authored by Hiramichi Hayashi and Yukia Hakuta.