The acetaldehyde-induced crystallization phenomenon was reported previously with the purpose of the improvement of the thermal shrinkage of oriented poly (ethylene terephthalate) (PET) film. This method involves dipping of amorphous PET film into acetaldehyde for only a short time, which causes a drastic increase in the film crystallinity. The toluene-induced crystallization was also described in detail for the solvent-induced crystallization, which did not give any whitening of the pet sample. In the present report, the origin of the thus-obtained high thermal resistivity of toluene-immersed PET film has been clarified on the basis of infrared ATR (attenuated total reflection) spectral measurement. In conclusion, the films which are extended at 79-83°C have shown extremely high mechanical strength at room temperature.