Aluminum oxide (AlxOy) films were deposited on the polyethylene terephthalate (PET) foils as barrier coatings using conventional DC magnetron sputtering, in which argon, oxygen and pure aluminum were used as a sputtering gas, reaction gas and target, respectively. The influences of oxygen and argon flux ratios on the surface morphology, stoichiometry and structure of AlxOy films were investigated. The mechanic and barrier properties of AlxOy/PET were discussed, too. Fourier transform infrared spectrum (FTIR) and X-ray photoelectron spectroscopy (XPS) investigations indicated that a strong relation between oxygen addition and compositions of AlxOy films, the aluminum oxide IR spectrum’s peak value, the intensity and the band width. Also, the Al—O expansion vibration peak’s wave number was proportional to the oxygen atom content in AlxOy. The studies about films barrier and mechanical characteristics demonstrated when the ratio of y and x in AlxOy film was closer to 1.5, the barrier property was better and horizontal, vertical elastic modulus and yield strength were greater. In addition, the mechanical characteristics of AlxOy/PET films significantly changed as the films were stretched, there were two significant turning points in stress-strained curve. 