The structure and properties (in both the molten and solid state) of polypropylene and the copolymers of propylene may be modified by gamma irradiation, which is frequently used for sterilization in diverse medical applications. Copolymers of propylene [poly-(ethylene-co-propylene) and poly-(ethylene-b-propylene)] nucleated with 1,3,2,4-di-para-methylbenzylidene (MDBS) were irradiated with γ rays. The radiation dose was 5 Mrad. The influence of radiation on the structure and properties was measured and is discussed. The molecular weight decreased to about 50% of the value of the nonirradiated samples [Mn(5 Mrad)/Mn(0 Mrad) = 0.51] while a slightly lower decrease of the molecular weight was observed for the samples nucleated with MDBS. For the irradiated polymers, in all cases, a decrease of the DSC determined melting temperature of the propylene and ethylene blocks was noted (ΔTm= 1°C to 5°C); on the other hand, for the ethylene blocks a certain increase in the crystallization temperature (1°C to 5°C) as a result of the gamma irradiation was ascertained. A significant decrease in the melt viscosity was noted (about 9 times, for the lower shear rates, and about 1.5 times, for the higher shear rates), as well as a decrease in the impact resistance due to the gamma radiation.