Thin films of poly(ethylene terephthalate) (PET) having a thickness of 100 mu m were exposed to different ion fluence of swift heavy ions of carbon in the range of 5 X 10(11) - 5 x 10(13) ions/cm(2). The effect of ion beam on structural and electrical modification has been studied by UV/vis, FTIR, X-ray diffraction (XRD), Differential Scanning Calorimetery (DSC), and AC electrical measurement techniques. On irradiation, a shift in absorption wavelength toward the red end of spectrum with increase of ion fluence was observed. The intensity of crystalline IR bands and main diffraction peak in XRD pattern were found to decrease with increase in ion fluence. It indicates the loss of crystallinity induced by ion-beam irradiation. The crystallite size was found to increase on irradiation. The melting temperature (T-m) of PET films increased at a low ion dose (5.0 x 10(12) ions/cm(2)), while it decreased at higher ion fluence (50.0 x 10(12) ions/cm(2)). The dielectric constant (epsilon') of PET films was increased with increase of ion fluence. The modifications brought about in the dielectric constant are correlated with chemical and molecular structural changes occurring on irradiation.