After investigating kinetics of the UV photodegradation of PET of film samples having a thickness of 4.4 mu m, we found that the photodegradation process takes place in two steps: a very rapid initial step followed by a normal step. This phenomenon is explained by using a concept of "weak links," We have obtained the rate constants of degradation of the "normal links" k(N), = 9.0 x 10(-7) h(-1) and "weak links" k(W) = 0.46 h(-1) and the number of scissions of weak links per molecules P-W = 0.22. For the samples treated by a UV stabilizer, we found k(N) = 2.0 x 10(-7) h(-1), k(W), = 0.11 h(-1), and P-W = 0.27. The ratios of the rate constants of the untreated to treated samples are 4.2 for k(W) and 4.5 for k(N). These results indicate that the UV stabilizer slows down the photodegradation rate of each step to the same extent, but hardly affects the number of scissions of weak links. Importantly, it is an implication that the lifetime of the PET thin film can be prolonged by a factor of 4.2 to 4.5 in the irradiation conditions used after being treated by the UV stabilizer.