Abstract: Optical smooth diamond films have been deposited at low temperatures (400~700℃) and their IR properties were studied for infrared optical coating applications.High density diamond nucleation was realized by a "Two Step Process" in which a high methane concentration was used for nucleation enhancement. However, biasing was shown insufficient for nucleation enhancement at low temperatures.Transmittance as high as 80% for diamond coated single crystal silicon wafer was observed.This level of IR transmission is good enough for practical applications of Si windows in a severe atmosphere.