Poly(ethylene naphthalate), PEN, irradiated by gamma -rays has been studied by positron annihilation lifetime (PAL). The PAL measurements were performed in the temperature range between 25 to 150 degreesC for three samples (non-, I and 5 Mrad gamma -irradiated). The ortho-positronium (o-Ps) lifetime, tau (3), is varying depending upon the phase of the polymer. The data clearly revealed the glass transition temperature, T-g, which is shifted toward lower values upon irradiation 5 Mrad, which could be attributed to the decrease in the degree of crystallinity. This is in consistent with the wide-angle X-ray scattering data, The thermal expansion coefficient values for the three samples indicate decrease in the crystallinity with increasing irradiation. To our knowledge no positron annihilation studies on PEN exist in the literature, so the results reported here may be considered new and important.