We present an ab initio calculation to further verify our earlier theoretical model. The result shows that the origin of the electrooptic effect in BaTiO3 crystal comes mainly from the displacement of the Ti4+ ions relative to the oxygen ions in the octahedron (TiO6) groups. The displacement of the Ti4+ ions lead to changes of the band wave functions of the electrons in the crystal, which produce the changes of the refractive indices. The calculation further reveals the physical meaning of parameters used in the theoretical model. (c) 2005 Elsevier B.V. All rights reserved.