KNbO3-based ferroelectrics are expected to be the most promising lead-free materials, which have been drawn much attention recently. The electronic structure and optical properties of KNbO3:Cu were studied with first principles. The calculated results under generalized gradient approximation are in agreement with experimental data, better than previous calculated values. The results show the considered peak of the absorption spectra is not just due to the single transition, and the Cu 3d states generate a peak at Fermi level. We predict the KNbO3 crystal will be metallic if the Cu-doping concentration is increased to be enough. (c) 2008 Elsevier B. V. All rights reserved.