The surface molecular orientation of biaxially stretched poly(ethylene terephthalate) (PET) films and the effect of the oxygen-ion and oxygen-radical beam irradiation on it have been investigated by using infrared-visible sum-frequency generation (SFG). The SFG spectra of PET surface showed a pronounced peak at 1705 cm(-1) which is characteristic of the C=O stretching vibration in an ester function. The SFG spectra indicate that the polymer chains at the surface appear to be well aligned parallel to the surface plane. The oxygen-ion beam exposure induces degradation of the polymer chain and converts the ester moiety to -COOH terminal species. In contrast to the case of oxygen-ion beam, oxygen-radical beam mainly affects the phenyl ring of PET.