Abstract Tetragonal BaTiO 3 under equibiaxial in-plane tensile stresses is investigated from first principles calculations. Our results show that this material transfers to the paraelectric phase at the critical stress of about 13 GPa. We also find the great enhancement of piezoelectricity close to the phase-transition region, due to large atomic displacements induced by stress. We also show that under the loading of applied stress, this material becomes markedly hard along the polar axis, especially at the phase-transition stress, but becomes soft along the nonpolar axes.