The production of fine metal oxide perticles in supercritical water has been studied. Cobalt nitrate solution and manganese nitrate solution have been selected as model solutions for metal salt aqueous solution. The particles of cobalt oxide and manganese oxide have been produced from the model solutions and the control of particle size and distribution has been tried with a change in reaction conditons. It was observed from the obtained results that the production of fine metal oxide particles in supercritical water was feasible and the dehydration rate was remarkably high in the surpercritical water. In spite of a short residence time (1-2 minutes), fine particles (2-3 μm) have been produced and it was suggested that the particle size distribution could be possibly controlled by a change in temperature.