The irradiation effect of gamma-rays on polypropylene (PP) has been studied through relaxational behaviour, using the positron annihilation lifetime (PAL) method. At low temperature, the intensity of a long-lived component of positronium, I-3, for unirradiated PP samples increased due to a termination of the local thermal motion of polymer structures. At 100 K, the increase in I-3 of 1 MGy gamma-ray irradiated samples became very small. However, the increase was observed again in the PP samples after 48 h of irradiation by positrons during the PAL experiment. This suggests a restructuring of the short polymer chains cut by gamma-ray irradiation may occur.