The diffraction efficiency of freestanding x ray transmission gratings is discussed in relation to the wavelength and grating period using both scalar diffraction theory and rigorous coupled wave analysis, and the grating design is accordingly optimized. A gold freestanding x ray transmission grating with period of 300 nm, line/period ratio of 055, thickness of 200 nm a size of 1 mm ×1 mm and fractional grating area of 65% is fabricated. The absolute diffraction efficiency of the fabricated grating was tested at the National Synchrotron Radiation Laboratory in the wavelength region of 55— 38 nm. The test result shows that the grating has a maximum efficiency of nearly 10%. The grating also shows a stable diffraction efficiency in the wavelength region from 15 nm to 35 nm.