We have made spectroscopic ellipsometric measurement to characterize the structure of diamond films in infrared region (2.5—12.5μm). These films are grown respectively by microwave plasma chemical vapor deposition (MPCVD) and hot filament chemical vapor deposition (HFCVD). It is found that the establishment of appropriate models has the strongest influence on the fit of ellipsometric spectra. The best fit is achieved for MPCVD film with a 77.5nm middle layer of SiO2, and for HFCVD film with a 879nm rough surface layer included by Bruggeman effective medium approximation. Finally, the refractive index (n) and the extinctive coefficient (k) are calculated for both films, which show that the film grown by MPCVD is optically better than by HFCVD apparently.