The ferroelectricity of BaTiO3 is investigated with the plane-wave pseudopotential method and the LCAO quantum chemical approach in the framework of density functional theory (DFT). Potential energy surfaces of various atomic displacements and the influence of lattice strain and lattice volume on the surfaces are examined. On the basis of the potential surfaces, phonon frequencies are also computed, which are in agreement with experiment results. The obtained potential energy surfaces show that the ferroelectric phase transition (from the cubic to the tetragonal phase) is decisively controlled by Ti displacement. The larger the lattice volume and the ratio c/a, the deeper the potential well. The calculated electronic populations and static charges show that from cubic to tetragonal phases Ti and O1 lose charges whereas Ba and O2 gain charges. The bond orders reveal that Ba-O has some extent of covalency, and during the phase transition the chemical bonding between metal and oxygen atoms, especially the Ti-O1 bond, is enhanced. Our results seem consistent with the vibronic theory about the origin of ferroelectricity of BaTiO3.