It was found that radiation resistances of polypropylene (PP) and copolypropylene (CPP) consisting 6% of ethylene unit far differ by molding conditions. And polymer quenched (quenching polymer) in methanol at -60℃ and water at 25℃ were more stable against irradiation compared with one quenched in cold press circulating water. In particular, effect of quenching was remarkable for the PP. From dynamic viscoelastic measurement, separation of α relaxation on crystal part and β relaxation on amorphous part were not clear for quenching polymer. On the contrary, then the separation of α and β relaxatins in the quenching polymer became clear by heat treatment, the radiation resistance of the polymer was greatly decreased. Thus, lower crystallinity of polypropylene in which separation of its crystal and its amorphous parts are not good, are favorable to materials of medical devices in the irradiation sterilization.