We have prepared a diamond membrane x-ray mask using our novel fabrication techniques which include fluidized bed pretreatment, stress control of diamond film, and chemical-mechanical polishing (CMP), and we have evaluated its important properties for practical use in detail. Surface roughness of diamond film was improved from 55.7 to 1.5 nm Ra using the CMP technique. The transmittance of visible light increased considerably due to the reduction of incident light scattering at the diamond surface, and the transmittance of 2.2-mu m-thick smooth diamond film was 92% at 633 nm. In-plane distortion-induced synchrotron radiation irradiation was Max(x,y)=(13,10) nm with a dosage of 12.8 kT/cm(2). In final evaluation, that is, of lithographic performance, we confirmed that a 0.2 mu m gate array pattern can be successfully transferred.