Predicting the refractive index based on the crystal structure has great significance in searching for new borate optical crystals. In this paper, we establish a method to calculate the anisotropic refractive indices of borate crystals with isolated planar BO(3) groups and network B-O skeletons. All the relative errors of the refractive indices in every direction between the calculated and experimental values are within 3.5%. The results show that our established method to calculate the isotropic refractive indices for the borate crystals is not only good for those crystals with isolated planar BO(3) group but also effective for those crystals with skeleton B-O nets. (C) 2010 Elsevier B.V. All rights reserved.