Diamond films deposited by hot filament chemical vapor deposition (HFCVD) under different deposition conditions are characterized using spectroscopic ellipsometric measurements in infrared region of 2.5—12.5 μm. Effects of deposition conditions on infrared spectroscopic ellipsometry of diamond films are investigated. The optical properties of diamond films depend strongls on its quality. The refractive index increases and the extinction coefficient decreases monotonicly with increasing sp3 C content. High quality optical diamond films are obtained under reactor pressure of 4.0 kPa when substrate temperature and carbon concentration are 750℃ and 0.9%, respectively. Refractive index and extinction coefficient of diamond film of high transparency are 2.385 and of the order of 10-4, respectively.