A technology of analyzing composition and optical properties of the diamond film deposited by MPCVD method is developed. Using infrared spectroscopic ellipsometer (IRSE) to analyze the composition and optical properties of the diamond film on Si substrate, the effect of different conditions of deposition process and annealing on the optical properties of the diamond film is studied. Experiments indicate that there are C-H bond, C=C bond, O-H bond and C=O bond in the diamond film. The growth conditions have an important effect on the content of C-H bond and C=C bond and refractive index of the diamond film. After annealing an obvious improvement of the optical properties of the diamond film is found.