Optical grade thick diamond films are polished on two sides by electron discharge machining (EDM) are with a mechanical method. The infrared transmittivities of Y2O3/diamond films are investigated in detail. Results indicate that the average infrared transmittivity is improved from 68.82% to 82.42% after deposition of Y2O3 at 10-μm central wavelength, and from 67.98% to 82.45% at 8–12-μm wave-band. The coated films meet the demands of infrared window applications. Therefore, the performance of the optical-class diamond thick films used in an actual infrared window can provide reliable application for basic research.