In the framework of the vibronic theory of ferroelectricity for the transparency region of tetragonal BaTiO3 components and of nonlinear optical susceptibility responsible for the second harmonic generation and the optical rectification are calculated. Selection rules for dipole transitions in the Γ point of the first Brilouin zone are extended over the whole zone. Calculations are confined to lowest order in expansion of susceptibilities in power series of low symmetry lattice distortion. Numerical values of susceptibilities have experimentally observed order of magnitude. The spectral behavior of and is calculated as well.