The effect of crystallinity of semi-crystaline polyethylene terephthalate(PET) on its macroscopic optical properties was studied by the spectroscopic ellipsometry.The PET films on silicon wafers were prepared by the spring method.Then the samples were irradiated by an infrared lamp and dried at 70～80℃ to remove the residual solvent.The annealing processes with different annealing temperatures and different periods of time (from 75℃ to 240℃

from 10min to 4h) were performed to obtained different crystallinity of PET films.The ellipsometric spectric spectra of these PET samples were measured by using a homemade automatic ellipsometer in the photon energy range of 4.3～6.2eV.The refractive index spectra and the extinction coefficient spectra of PET films were obtained by fitting the experimental data of ellipsometric spectra with the theoretical calculation based on a monolayer model.Some thick PET film samples with corresponding annealing treatments were characterized by the X-ray diffraction to check the crystallinity. Our results shou that the optical constants of PET thin films increase signficantly with increasing crystallinity and incline to those of crystalline state.There are peaks of the refractive nidex and the extinction coefficient in our spectra.The optical constant spectra of biaxially-stretched PET were also measured for comparison.The absorption edge is near 4.2eV for both kinds of PET

but the positions of the peak of the extinction coefficient are different.The micro-crystal dimension may also be concerned with the change of optical constants.