The aluminum oxide thin films have been deposited on PET substrate film using r.f. magnetron sputtering from an aluminum oxide magnetron target and argon plasma. The electronic structures of aluminum oxides have been using XPS. The XPS spectrum for the aluminum oxide showed the presence of a well resolved Al 2p spectral line at 74.3 ±0.2 eV (FWHM=∼2.1). Depending on synthetic conditions, the binding energies of Al 2p and O 1s for samples used in the current study are same, or very similar. The average ratio of Oo/Al for all the aluminum oxides was 2.8, indicating that the deviation from the stoichiometric atomic ratio of 1.5 in Al₂O₃ is accounted for by the incomplete oxidation. Given the reference energies of 531.0 eV for O 1s and 72.7 eV for Al 2p, the Al 2p peak tended to shift to higher energies while the O 1s peak shifted to lower energies. During the synthesis of aluminum oxide thin films, there was a correlation between the preparative conditions and the relative intensity for the Al 2p and O 1s peak. The peak intensities of Al 2p and O 1s were increased with increasing the sputtering power as well as the sputtering time. These results indicate that there was a linear relationship between the peak intensities of Alo 2p and O 1s and the sputtering power.