A method of manufacturing a nano metal grating is provided. In the method, a patterned metal oxide film is formed on a surface of the metal layer through the formation of metal oxides in an oxygen ashing process, and the nano metal grating is manufactured by using the patterned metal oxide film as a mask. Thus, a drawback that the metal layer cannot be etched after the metal layer is oxidized is solved, and the metal oxide film is not only used as a mask to manufacture the nano metal grating, but the metal oxide film is also used as a protective layer of the nano metal grating.