Abstract: By combining the reality of CVD diamond,transmittance model of free standing diamond films was derived.In the model,several factors affect transmittance such as chromatic dispersion, optical non-uniformity, surface roughness and absorption of the graphite （free carries） and carbon-hydrogen bonding in the sample-were considered.IR spectra of diamond was non-linear least square fitted（NLSF） by Levenberg-Marquardt algorithm.Reliable frequency dependence of n and k, together with thickness,surface roughness,impurities and microstructure were then obtained.This result will be important for a correct analysis of the IR properties of diamond films.