Disclosed is a method for preparing a metal oxide solid solution in nano size. The metal oxide solid solution is prepared by reacting a reactant mixture containing water and at least two water-soluble metal compounds at 200 to 700.degree. C. under a pressure of 180 to 550 bar in a continuous manner, wherein the reactant mixture contains the metal compounds at an amount of 0.01 to 30% by weight in total and the solid solution has a crystallite size of 1 to 1,000 nm. The metal oxide solid solution is, in particular suitable as a UV light shielding agent or as an oxygen storage component.