The viscoelastic properties of Gamma-ray irradiated polyethylene terephthalate (PET) sheets, extruded in the glassy state, were measured by a method of forced resonance oscillations. Density variations due to γ-irradiation with different doses were found suitable to describe changes of the cross-linking rate. The dynamic Young's modulus of PET-films showed a decrease within the dose range 200-500 kGy which has anomalous behaviour related to the glass transition (alpha-relaxation) by γ-irradiation to a rubberlike state at a dose of 200 kGy. The low frequency velocity of sound showed no correlation with the density of irradiated PET-films.