In order to evaluate the effect of Er doping in the range of 0-1.0 mol% on optical indirect band gap energy (E-g) of the film, the Er-doped TiO2 (Er-TiO2) thin films were spin-coated onto fluorine-doped SnO2 coated (FTO) glasses. Glancing angle X-ray diffraction (GAXRD) results indicated that the films whose thickness was 550 nm consisted of pure anatase and FTO substrate. The anatase (101) TiO2 peaks became broader and weaker with the rise in Er content. The apparent crystallite size decreased from 12 nm to 10 nm with increasing the amount of Er from 0 mol% to 1.0 mol%. UV-vis spectrophotometry showed that the values of E-g decreased from 3.25 eV to 2.81 eV with the increase of Er doping from 0 to 0.7 mol%, but changed to 2.89 eV when Er content was 1.0 mol%. The reduction in E-g might be attributed to electron and/or hole trapping at the donor and acceptor levels in the TiO2 band structure. (C) 2013 Elsevier B. V. All rights reserved.