The biaxially drawn PET film was aminolyzed with 5.64%, 10.72% and 16.34% aqueous guanidine solution at various temperature for various temperature for various time, and its properties were investigated with some physico-chemical methods. The results are as follows; The viscosity and the molecular weight of aminolyzed PET were decreased and degree of degradation was increased with duration of aminolysis time. The heat of fusion and the intensities of 'trans' bands(848, 972, 1372 and 1471cm-1), crystal benzene mode(1386 cm-1) and true crystal band (1023.7cm-1) were increased while the intensities of 'gauche' bands(898, 1042, 1102, 1173, 1370 and 1454 cm-1) were decreased with duration of aminolysis time. We also obtained IR spectrum of isolated crystalline region for PET film aminolyzed with 5.64% aqueous guanidine solution for 24 hours by subtracting it absorbance spectrum from the reference absorbance spectrum of PET. This spectrum did not show bands for guche conformation and folded region but only bands for trans conformation and true crystalline region. The rate of aminolysis for amorphous region(ka) was larger than that for crystalline region(kc). The activation energy for aminolysis of amorphous region (△Ea) and that for aminolysis of crystalline region(△Ea) were 22.03 Kcal/mole and 21.08 Kcal/mole, respectively. From these results, it was concluded that the amorphous region of PET was aminolyzed more selectively than the folded and crystalline region, and the folded region was also aminolyzed when treated for long time.