The effects of low (1.0, 3.0 keV) and high (1.0 MeV) energy reactive oxygen ion beam irradiation on surface nano-morphology and chemical state of the constituent elements in the surface layer of a polyethylene terephthalate thin him (thickness 25 mum) are studied in this paper. PET thin film, which has a high potential as a material for biomedical and electrical industries, was irradiated by oxygen ions of variable doses. The treated films were investigated by Atomic Force Microscopy (AFM) and X-ray Photoelectron Spectroscopy (XPS). AFM observation revealed that an increase in dose increased the surface roughness. The significant changes in the chemical composition of the surface layer were confirmed by XPS quantitatively. The scission of the chains in the surface layer of PET film was induced by oxygen ion beam irradiation, and the atomic ratio (O1s/C1s) was decreased. The selective sputtering of oxygen atoms in PET film was occurred. This phenomenon is responsible for the creation of carbon-rich surface layer.