Silicon incorporated diamond-like carbon (Si-DLC) films were deposited by 13.56 MHz r.f. plasma assisted chemical vapor deposition (RF-PACVD), and the optical properties of the films were examined by spectroscopic ellipsometry analysis. In particular, the dispersion model developed by Jellison and Modine was used to determine the optical constants by fitting the measured ellipsometric spectra in the 300-1200 nm range. Various mixtures of benzene and diluted silane (SiH4:H-2 = 10:90) were used as reaction gases, and the Si concentration in the film was found to increase monotonically up to 17 at.% as the diluted silane fraction in the reaction gas was increased from 0 to 95%. However, the variation of the optical gap exhibited distinctive silicon-concentration dependence; the minimum optical gap was found from the film with 5 at.% silicon concentration. The observed changes of the optical properties were attributed to the modification of the atomic bonding structures induced by silicon incorporation. In particular, the change in SP2-bonded carbon cluster sizes and the interlinking between SP2 clusters are discussed.