CO2 laser-irradiated polyester (PEs) films containing quinacridone pigments (QA) fluoresced yellow under irradiation of UV light (365 nm). In this study, X-ray photoelectron spectroscopy (XPS) and thermal analysis were used to clarify the mechanisms of this fluorescence phenomenon. The pyrolysis temperature of PEs was lower than that of QA, and the fluorescence was reproduced on the PEs film containing QA by heating over the pyrolysis temperature of the PEs for even a short period. These results suggested that the fluorescence phenomenon be due to the thermal decomposition of PEs. The amounts of carboxyl groups on the surface of PEs after CO2 laser irradiation measured from XPS were correlated with the intensity of fluorescence. In addition, the amounts of carboxyl groups on PEs containing QA were approximately a half of those on non-containing PEs. In consequence, the mechanism of the fluorescence phenomenon could be explained as a following structure change : Some of carboxy groups formed in the pyrolysis of PEs by CO2 laser irradiation would bond to QA.