Electronic structures of the (001) surface of cubic phase barium titanate have been calculated by using plane-wave pseudopotential method within the density functional theory. Geometry optimization indicates that the largest atomic relaxation occurs to metal atoms in surface layers where all atoms are displaced inward, and the interlayer distances vary alternately. Total energy calculation shows that the TiO2-terminated surface is unstable compared with that with BaO termination. One reason is that surface state of O-2p orbital observed in band structure of TiO2 termination helps the electron state shift towards higher energy region in both valence band and conduction band. Redistribution of electrons near surface region in TiO2 termination also provides evidence that intensity of covalent bonding between Ti and O atoms differs and hence TiO2 termination is more suitable for surface adsorption. In contrast, such difference was eliminated in BaO termination, accounting for its superiority in surface stability.