The radiation stabilities of high- and low-molecular-weight polypropylene (PP) with and without nucleating agent (NA) are compared. It is found that PP with NA are less stable during irradiation than PP without NA, especially the lower-molecular-weight PP. In order to correlate these results with their morphology, haze measurements, differential scanning calorimetric analysis, dynamic mechanical relaxation studies, chemiluminescence analysis, pulsed nuclear magnetic resonance analysis and micrography of the PP have been carried out. In both cases (high- and low-molecular-weight PP), the addition of NA increased the transparency and peak crystallization temperature of the PP. These qualities are an advantage in the production of medical disposables. The high transparency and peak crystallization temperature in the PP with NA were found to be due to their small spherulite size. In the high-molecular-weight PP, the spherulite size of the PP without NA was 1.5 times larger than the PP with NA, while in the lower-molecular-weight PP, it was 5.0 times larger. From the dynamic mechanical relaxation studies and pulsed nuclear magnetic resonance analysis of the high-molecular-weight PP, the unstable PP with NA was found to be more crystalline than the PP without NA. Accordingly, a higher concentration of peroxy radicals, which cause the oxidative degradation of PP, was found in the PP with NA than in the PP without NA. In the low-molecular-weight PP, the PP with NA was also found to be more crystalline that the PP without NA from their dynamic mechanical relaxation studies. In contrast, the chemiluminescence measurements showed that a higher concentration of peroxy radicals was found in the PP without NA than in the PP with NA. Two reasons were given to account for this discrepancy. From pulsed nuclear magnetic resonance analysis, at the temperature at which the chemiluminescence measurement was carried out, the PP without NA was found to be more crystalline than the PP with NA. The other reason given was due to a difference in spherulite size between these PP.