Thermal properties and degradation behavior of the PET/PBT blends prepared by solotion-precipitation method and the PET-PBT copolymers produced by the interchange reaction were studied by differential scanning calorimetry and thermogravimetry. The activation energies for the thermal degradation of PET/PBT blends and PET-PBT complymers showed the lowest value at a certain composition, but the additivity of composition of the constituent was held for the Gibbs free energies.