Effects of solvent treatment on the structure of PET film and the dyeing transition temporature were studied. We selected benzyl alclhol, formic acid, dimethyl formamide and dimethyl sulfoxide because of their similar solubility parameters to that of the aliphatic residue of PEI. The PET films treated in the solvents showed an increase in custallinity but a decrease in density compared with the original films. The abnormaly low density could be introduced from the internal voids formed during crystallization. A linear relationship was observed between the depth of void formation and square root of the time for solvent treatment. Data for dye uptake dependence of temperature were curve-fitted using 4th order polynomial regression program. The curve obtained was used for the computation of dyeing transition templerature(T_d ) and the calculated value was compared with the value obtained by graphical methods. The T_d values acquired using different methods showed a good correlation(correlation=0.91). T_d oF the film treated with solvent decreased an Increase in treating time.