Abstract: The infrared transmittanoe of high resistivity GaAs monoorystal and the plasma anodized film on GaAs substrate are measured by using an IR-450 S type infrared spectrophotometer. The experiments show that, in the range of 2.5-13.6 μm wavelength, the transmittance of GaAs monocrystal with thiohness of 0.70 mm is greater than 50%. In the range of 2.5-10 μm wavelength, the anodized film with thickness of 0.590 un has some anti-reflection effect on GaAs. In 10-26 μm wavelength, a strong and wide infrared absorption band is observed. This absorption band can be utilized to examine plasma anodized film on the GaAs wafer.