- Thermally induced dynamics of interphase boundaries are considered in confined ferroelectrics at a first-order phase transition. The size dependence of the temperature of the stability limit of the paraelectric phase and the phase transition temperature is calculated for a semi-infinite plate with one finite size (thickness). The velocity of the interphase boundary is presented as a function of its thickness for the thermally induced motion. The result is in agreement with the experiment on PbTiO3. It is shown that close to the phase transition the interphase boundary should change the direction of its motion at a critical thickness of the sample, reflecting the first-order character of the ferroelectric phase transition.