The effects of lamination temperature on the properties of poly(ethylene terephthalate-co-isophthalate) (co-PET), such as thermal, mechanical, and barrier properties, have been investigated in co-PET laminated steel. The variation of the degree of crystallinity and the orientation of the co-PET film during the lamination process was examined using DSC, X-ray diffraction, and birefringence, and water vapor permeability was also measured with varying lamination temperature. Both the degree of crystallinity and the orientation of the co-PET film decreased and water vapor permeability increased with increasing lamination temperature. The stress-strain curves of the co-PET films were different, depending on the lamination temperature. The stress in the co-PET film laminated at higher temperature was lower at a given strain, due to the increase of the amorphous region. The effects of annealing temperature and the extent of drawing on the residual stress in co-PET/tin-free steel (TFS) joints were investigated by examining the stress relaxation behavior of co-PET. It was necessary to heat co-PET/TFS joints at more than 150 degrees C in order to eliminate the residual stress.