We have studied the technologies for the fabrication of a diamond/SiC double layer for use as an x-ray mask membrane. Each film layer (diamond and SiC) has excellent quality, i.e., crystallinity and purity, and is a precisely stress-controlled freestanding membrane of the diamond/SiC double layer. We have fabricated the diamond/SiC double layer membrane and have also evaluated important properties required for the use of the membrane as an x-ray mask in detail. Surface roughness of the diamond layer was made to be 7 nm nms using the chemical-mechanical polishing technique. Due to the smoothing, the amount of incident light scattered at the diamond surface was decreased, and the visible light (lambda = 633 nm) transmittance of the diamond/SiC double layer membrane (film thicknesses : diamond/SiC = 1.1 mu m/0.3 mu m) attained a maximum value of 64%. It was confirmed by transmission electron microscopy observation that the diamond/SiC boundary layer had a coexistence region, so the double layer had strong adhesion. Furthermore, we evaluated the biaxial Young’s modulus using the bulge method, which was 1070 GPa for the diamond/SiC double layer.