Diamond film is an ultra-durable optical material with high thermal conductivity and good transmission in near-infrared and far-IR (8-14 mu m) wavebands. CVD diamond is subjected to oxidation at temperature higher than 780 degrees C bared in air for 3 min, while it can be protected from oxidation for extended exposure in air at temperature up to 900 degrees C by a coating of aluminum nitride. Highly oriented AlN coatings were prepared for infrared windows on diamond films by reactive sputtering method and the average surface roughness (R-a) of the coatings was about 10 nm. The deposited films were characterized by X-ray diffraction (XRD) and atom force microscope (AFM). XRD confirmed the preferential orientation nature and AFM showed nanostructures. Optical properties of diamond films coated AlN thin film was investigated using infrared spectrum (IR) compared with that for as-grown diamond films. (c) 2006 Elsevier B.V. All rights reserved.