The performance prediction of freestanding blazed transmission grating is discussed using scalar diffraction theory, and grating design parameters are accodingly given. A freestanding transmission grating with a period of 1 m, duty cycle of 0.10.2, aspect ratio of about 100, thickness of 10 m and fractional area of 65% is successfully fabricated by holographic lithography and anisotropic wet etching. The size of a single die is 15 mm 15 mm divided into four 5 mm 5 mm windows. The diffraction efficiency of the fabricated grating is tested at the National Synchrotron Radiation Laboratory in a wavelength region of 550 nm. The wavelength scanning results show a strong blazing effect in the direction of specular reflection from mirror-like grating sidewalls, as expected. The measured and normalized diffraction efficiency is consistent with the theoretical prediction within 38%49% due to grating structural imperfections. These experimental results prove not only the concept of blazed transmission grating but also practicability of the fabrication process.