A new approach for accurate measurement of the absorbing film thickness and optical constants by combined ellipsometry and spectrophotometry is introduced. The details are studied in terms of the mathematical models and formulats in comparison with the commonly used method which is only dependent on the ellipsometry data specifically. Using variable angle spectroscopic ellipsometry (VASE), the diamond-like carbon film deposited by a glow discharge technique was characterized at the wavelength range of 250—1700 nm. The results indicate that, generally, it is rather difficult to determine the thickness and optical constants of absorbing film accurately and rapidly due to the strong correlation between thickness and n, k. By simultaneously fitting ellipsometry parameters and film transmittance, however, the optical constants could be obtained easily and accurately from a unique solution without any predetermined dispersion model. The fitting results of DLC show that this approach is a promising method to determine the thicknesses and optical constants of films, especially of the thin absorbing films.