Abstract
We present a model calculation of phenomenological theory for the structural phase transition at Ta=222 K of hexagonal BaTiO3 under uniaxial pressure. It is shown that the mm2 phase below Ta becomes unstable against formation of the 222 phase with decreasing temperature and a discontinuous phase transition from mm2 to 222 occurs at some temperature under the pressure along [100] axis because of the contribution of the sixth order terms of the soft mode amplitude.