Abstract: A technique for bonding an organic group with the surface of fine particles such as nanoparticles through strong linkage is provided, whereas such fine particles are attracting attention as materials essential for development of high-tech products because of various unique excellent characteristics and functions thereof. Organically modified metal oxide fine particles can be obtained by adapting high-temperature, high-pressure water as a reaction field to bond an organic matter with the surface of metal oxide fine particles through strong linkage. The use of the same condition enables not only the formation of metal oxide fine particles but also the organic modification of the formed fine particles. The resulting organically modified metal oxide fine particles exhibit excellent properties, characteristics and functions.