A probability model (PM) developed recently, which is an alternative to the conventional Havriliak-Negami (HN) dispersion function for polymer systems, is applied for the first time to the study of the dielectric and -relaxation of amorphous polyethylene terephthalate (PET). It has been found that the temperature dependence of the maxima of the imaginary part of the dielectric function and of the half-width of the distribution in the PM of the -relaxation exhibit Vogel-Fulcher-type temperature behaviour; the temperature dependence of the maxima of the imaginary part of the dielectric function of the -relaxation is proportional to the ratio of the gauche content to the trans content of the ethylene glycol linkages in PET. Finally, the mechanisms of the and -relaxation are elucidated concisely.