The diamond has very excellent optical quality, with very high transmittivity and refractivity in most spectrums of the wave bands, and also the excellent thermal, chemical, mechanica1 properties. So it makes the CVD (Chemical Vapour Deposition) diamond films be used as the protection films of various optical lenses. Especially, the radar wave is hardly distorted while passing the diamond films. It can also solve the problems to which the cover of radar is subjected due to the high-speed raindrops or dust and the sudden high temperature surge in the high-speed flight. Therefore the study on the optical characteristic of the diamond films is thought to have much practical value and national defense meaning. This paper has done a basic research on the application of diamond thick films in the optical windows. From depositing the diamond thick films and polishing the films, to evaluating the infrared transmittivity, the thesis has carried out systematic theoretical analysis and experiments, obtaining an important result in initial stage. The main work includes: 1. Large area high quality freestanding diamond thick films have been prepared by DC arc plasma jet CVD apparatus. The influence of the deposition actors upon the diamond thick film quality is analyzed, and the deposition condition on which the high quality diamond thick film can be produced is acquired. 2. Polishing technologies for polishing CVD diamond thick film have been reviewed, and mechanical polishing methods are chosen according to the actual condition. Through plenty of polishing experiments, this thesis analyzes the reasons why the crystal’s nucleating plane and the crystal’s growing plane of diamond thick films have different polishing results. 3. Infrared spectrometer is adopted to measure the transmittivity of the silicate glass which have different surface roughness, obtaining the change rule of the surface roughness to the infrared transmittivity of the silicate glass. By measuring the infrared transmittivity of the silicate glass which have different thickness values, the results of the influence of thickness to the transmittivity of the silicate glass is obtained. The conclusion of the research can shed light on the infrared transmittivity experiments of CVD diamond thick films in future.