We proposed the use of a double layer structure consisting of diamond and SiC films as an x-ray mask membrane. The SiC film was fabricated by a radio frequency magnetron sputtering method on polished Si wafer and diamond was fabricated by magnetically enhanced microwave plasma chemical vapor deposition on the Si wafer covered with SiC film. Diamond film fabricated on Si wafer had high crystallinity and purity; however, it also had strong compressive stress. As a result of the combination of diamond film with strong compressive stress and SiC film with strong tensile stress, we obtained a freestanding membrane of diamond/SiC double layer structure. The stress in the-double layer film was an average of the stress of the individual layers of diamond and SiC on the Si wafer. The double layer technique enabled us to use high quality diamond as the main layer of the x-ray mask membrane regardless of its film stress.