Optical properties (transmission and refractive index) and chemical structure (XPS) of diamond-like coating (DLC) film before and after focused ion beam milling (FIBM) process, were investigated in this paper. It is shown by our FIBM experiment that influence of FIBM on Optical properties and chemical structure of the DLC film is not apparent, especially in near-infra region. The film still can be used for optical applications. As an application example, diffractive optical element (DOE) was directly fabricated on the film by use of FIBM. Measured diffraction efficiency of the DOE is 73%, which is acceptable for conventional use. / Singapore-MIT Alliance (SMA)