We studied the effects of 172 nm Xe₂* excimer lamp irradiation on polyethylene terephthalate (PET) surfaces. Two kinds of techniques were applied: vacuum ultraviolet (VUV) light irradiation and VUV irradiation in the presence of oxygen gas (VUV/O₃). The modified PET surfaces were investigated by using contact angle measurements which enabled the surface free energy to be calculated, X-ray photoelectron spectroscopy (XPS), nano-thermal analysis (nano-TA), and atomic force microscopy (AFM). The surface free energy increased significantly after the treatments. The results of XPS analysis showed that the elemental ratio of oxygen on the surface increased, whereas that of carbon decreased. From the deconvoluted C1s and O1s spectra, it was revealed that new oxidized functional groups such as alcoholic and carboxyl groups were generated. The nano-TA results showed that a low melting temperature (T_m) layer had formed on the VUV and VUV/O₃ treated PET surfaces. The results of AFM measurements showed there were no remarkable changes after the treatments compared with untreated PET. In summary, the VUV and VUV/O₃ treatments using a Xe₂* excimer lamp not only change the surface functionalities but also reduce the T_m of the PET surfaces without significantly affecting the surface morphologies.