The electronic band structures of SrBi2Nb2O.9 (SBN) in both the ferroelectric and paraelectric phases were calculated using firstprinciples method. The paraelectric phase is of an indirect band gap of 1.57eV ; whereas the ferroelectric phase is of a direct band gap of 2.23eV. The valence band maxima of both the paraelectric and ferroelectric phases are mainly contributed from O2p states; while the conduction band minima are from Nb4dand Bi6p states. When transforming from the paraelectric to the ferroelectric phases, the strong hybridization between Nb4d and O2pand BiO(2) hybridization enhance the distortion of NbO6poctahedra and reduce the total energy of the system,in favor of stabilizing the ferroelectric phase. The calculation shows that the low leak current in SBN is related to Bi6p state.