The phase transformations of NaSbO3 and NaBiO3 from ilmenite phase to the orthorhombic phase are predicted within the density functional theory (DFT) using the generalized gradient approximation. The bonding properties of the covalent band maximum and the conduction band bottom are used to elucidate the comparatively large band gap in the ilmenite phase to the orthorhombic phase. Bond populations and atomic electronnegativities are used to explain the comparatively small energy gap of NaBiO3 to NaSbO3. The abnormal phenomenon that the band gap increases with the increasing of hydrostatic pressure is explained to be more distorting during the pressure increase. (c) 2005 Elsevier B.V. All rights reserved.