The C=O stretch vibration (ν_C=O) of the polyethylene terephthalate (PET) had been studied by fourier transform attenuated total reflection infrared spectroscopy (ATR-FTIR) and ATR two-dimensional infrared spectroscopy.The νC=O occurred respectively near 1700 cm^-1, 1709 cm^-1 and 1718 cm^-1(below glass transition temperature), and the sequence of ν_C=O intensity changes was 1709 cm^-1> 1700^-1 > 1718 cm^-1 with the increase of temperature (313 -343 K). But the ν_C=O were observed near 1709 cm^-1 and 1718 cm^-1(above glass transition temperature), and the sequence of ν_C=O intensity changes was 1718 cm-1>1709 cm-1 with the increase of temperature (353 -393 K). The paper demonstrated the key roles of ATR two-dimensional infrared spectroscopy in the analysis of thermal denaturation of the PET.