According to M.W. distribution, the thermal properties and microstructure of the PET blends which made of the different intrinsic viscosities have been studied. The M.W. distributions and the thermal properties were obtained by GPC and DSC, the crystallinites by the chance of densities of PET film and the crystabite size by x-ray diffracter. The results obtained are as follows; The intrinsic viscosities of the PET blends were approched to arithmetic mean. The M.W. distributions of the PET blends were broadened and the number average M.W. were less than the pure PET. The TR of PET blends were decreased and the heat of fusion were increased compared with the pure PET. The rate of crystallization and ultimate crystallinity of the PET blends were higher than the pure PET.