Polyethylenterephtarate (PET) films with deposited Al or SiO_x thin films are utilized for a wide variety of package materials. The interfacial interaction between PET films and vapor deposited thin films was investigated using X-ray Photoelectron Spectroscopy (XPS). We studied some influences of the vapor deposition process on the surfaces of PET films, using commercial samples. In order to get the information at the interface between vapor deposited thin films and PET films, analysis of PET surface structures was carried out by breaking samples in the vicinity of the interface. The XPS data, the full width of half maximum (FWHM) of C-C bonding peaks and the changes in the peak area of functional groups obtained by means of peak separation, suggested that the surface structure of PET films was influenced by vapor deposition process, and Si-C bonding was formed at the interface.