Supercritical water can provide an excellent reaction environment for hydrothermal crystallization of metal oxide particles. Because of the drastic change of properties of water around the critical point, density, dielectric constant, and ionic product, the phase behavior for the supercritical water-light gas (Oz, Hz, etc.) system and reaction equilibrium/rate can be varied to synthesize new materials or define particle morphologies. In this work, hydrothermal crystallization experiments were performed with several types of flow apparatuses that allow convenient manipulation of variables such as temperature, pressure, and residence time. The proposed supercritical hydrothermal synthesis method has the following desirable features: (1) ultrafine particles can be produced, (2) morphology of the produced particles can be controlled with small changes in pressure or temperature, and (3) a reducing or oxidizing atmosphere can be applied by introducing oxygen, hydrogen, or other gases. An overview of this method is given for functional material synthesis of significant industrial interest including barium hexaferrite magnetic particles, YAG/Tb phosphor fine particles, and lithium cobalt fine crystals.