Silicon nanocrystals (nc-Si) quantum dots is one of the most promising and important fields in the research and application of Si luminescence nowadays. The nc-Si embedded in SiO2 thin film is grown on Si substrate by using thermal evaporation method. Strong photoluminescence of nc-Si is observed. The films are investigated by spectroscopic ellipsometer in the visible region at room temperature. Employing the effective-medium approximation (EMA) model combined with the Lorentz oscillator model in the ellipsometric parameters fitting,the optical constants of nc-Si with the sizes of 3 nm and 5 nm are obtained,respectively,in the spectral region from 300 nm to 830 nm. The data obtained will be helpful for the design and manufacture of Si-based micro-nanophotonic devices.