Electronic structures and optical properties in the core-level spectra of strontium titanate are investigated by the first-principles under scissor approximation. The direct- and indirect-band gaps are equal to 2.24 and 1.87 eV, respectively. The complex dielectric function and optical constants, such as optical absorption coefficient, refractive index, extinction coefficient, energy-loss spectrum and reflectivity, are calculated, which are in good agreement with the experimental data, better than those from linear muffin-tin orbital method. We also have interpreted the origin of the spectral peaks on the basis of the electronic band structures. (C) 2004 Published by Elsevier B.V.