Structural changes and dynamic mechanical properties of istotropic high-density polyethylene (HDPE) for both virgin and gamma-ray irradiated samples of different thickness, have been studied. The structural changes due to irradiation in the presence of air were studied by using infra-red spectroscopy. The radiation induced radicals were not post-irradiation annealed. The effects of these changes on the mechanical behaviour were investigated by using a modified dynamic resonance technique. It was found that the polymer chain crosslinking rate, as a result of irradiation by gamma-ray doses up to 200 kGy, depends on the sample thickness. However, for irradiation doses above 600 kGy, polymer chain scission is the most dominant phenomenon and is independent of the sample thickness.