Diamond is a very attractive material for x-ray mask membranes since it has high Young’s modulus, high transmission under exposure to soft x rays (in the exposure wavelength region of 0.5-1.5 nm), and good thermal conductivity. However, there remains a drawback of synchrotron radiation damage. Microwave plasma chemical vapor deposition diamond membranes under several kinds of deposition conditions have been evaluated. It is found that x rays attack only the nondiamond component of the membrane, and a diamond membrane having good crystallinity shows high x-ray irradiation durability.