The surface molecular conformation of poly(ethylene terephthalate) (PET) and the interface between alumina and PET has been investigated by sum-frequency generation (SFG). A considerable improvement in adhesion strength between AlO_x and PET was achieved by short time Ar plasma modification. The increase in adhesion strength shows good correlation with the increase in SFG peak strength. By depositing AlO_x, increase of SFG intensities and appearance of a new peak at 1,685 cm^-1 are observed, indicating the formation of a C=O···Al bond at the interface. Adhesion improvement by the Ar plasma treatment is not due to the increase in the C=O···Al chemical species at the interface, but due to the increase in the surface energy followed by the increase in the area of the homogeneous amorphous region of the PET and the creation of the surface hydroxyl groups induced by the plasma pretreatment.