Laser ablation of a poly(ethylene terephthalate) (PET) film with an excimer laser beam, 248 nm, was analyzed by various analysis methods, and the mechanism was studied. The ablation was carried out in an argon gas atmosphere at room temperature. Produced gases and some fragments were collected and analyzed by GC-MS. The photochemical fragmentation of PET through the Norrish type I reaction was dominant, but above the threshold fluence, ca. 30 mJ/cm(2), thermal reactions such as cyclic oligomerization occurred and photochemical fragmentation was accelerated by the thermal energy.