Diamond-like carbon film (DLC) is well known for its optical, electrical and mechanical properties. However, its properties can vary widely from graphite to diamond-like, strongly depending on the film preparation. So the determination of sp3/sp2 ratios of the DLC films is very important. In this work, a series of DLC films have been prepared on silicon substrates by Filtered Arc Deposition (FAD) and an optical method - spectroscopic ellipsometry (SE) has been applied to study these samples in the visible wavelength range. In the analysis of SE spectra, a single DLC layer on Si substrate model has been used and the DLC layer simply consists of sp3, sp2 and void constituents. The Bruggeman effective medium approximation has been applied to calculate the dielectric response of these three components. From the interpretation of SE spectra, the film thicknesses and the volume fractions of sp3, sp2 and void have been derived. SE results show: the fitted film thicknesses are consistent with those of Rutherford Backscattering spectroscopy; the fractions of sp3 correlate wit results of electron energy loss spectroscopy studies; for all these samples, the concentrations of void are very small which implies the compact DLC layers. It shows that SE is a very useful and promising optical method to determine the sp3/sp2 ratios of the DLC films.