Abstract: Diamond is good optical protective material for its excellent transmittivity in infrared range. Moreover, diamond has advanced mechenical、 thermal and chemical properties, which can be used as protective films coated on other infrared devices. We have deposited diamond thin films on Si wafer. The infrared reflection of Si wafer coated diamond film is reduced in the wave range of 2.5～15 μm, and its maximum increment of transmittivity is from 52% to 64.1% at 8 μm.