Spectroscopic ellipsometry is a non-distructive spectroscopic technique. To find the optimal Microwave Plasma Chemical Vapour Phase Deposition (MPCVD) condition of diamond thin films, the infrared optical properties of diamond thin films are characterized by using an infrared spectroscopic ellipsometric spectrometer. The influence of substrate temperature and chamber pressure on the infrared optical properties of diamond thin films is analyzed. It is found that the optimal infrared spectroscopic ellipsometric properties and an average refractive index of 2.393 can be obtained for the diamond thin films with a constant methane concentration, a substrate temperature of 750℃ and the chamber pressure of 4.0kPa. The result shows that the optical properties of diamond thin films are closely related to their quality.