The optical properties of TiO2 thin films prepared by electron beam evaporation were studied by spectroscopic ellipsometry and analyzed quantitatively using effective medium approximation theory and an effective series capacitance model. The refractive indices of TiO2 are essentially constant and approach to those of bulk TiO2 for films thicker than 40 nm, but drop sharply with a decrease in thickness from 40 to 5.5 nm. This phenomenon can be interpreted quantitatively by the thickness dependence of the void fraction and interfacial oxide region. The optical band gaps calculated from Tauc law increase with an increase of film thickness, and can be attributed to the contribution of disorder effect. (C) 2013 Elsevier B. V. All rights reserved.