Depolymerization reaction of PET (Poly Ethylene Terephthalate) was analyzed by both experimental data and calculated data obtained from molecular orbital method. Ethylene Glycol and another alcohol with long carbon chain are used as a solvent to search the reaction mechanism. The activation energy obtained from the optimized structures of ground and transition states was compared with our experimental data. According to these comparisons, the ring formation with alkoxyl group and carbonyl carbon at transition state is more important when carboxyl group of PET is depolymerized.