Recently, ␥-irradiation of ultrahigh molecular weight polyethylene (UHMWPE) to high doses such as 1 MGy was shown by Oonishi et al. to be very effective in improving the wear resistance of UHMWPE. The present work was undertaken to characterize the UHMWPE irradiated with ␥-rays and electron beams to doses ranging from 25 KGy to 5 MGy. The radical concentration was determined by electron spin resonance (ESR) spectroscopy, while infrared spectroscopy was used to study the carbonyl and double bond formation on the irradiated UHMWPE. The change of the melting temperature of UHMWPE upon irradiation was examined by differential scanning calorimetry, while mechanical tests were performed to measure the compressive creep deformation, surface hardness, and tensile strength of the irradiated UHMWPE. The UHMWPE irradiated with ␥-rays in air and then stored in air showed that the decrease in polymer radical concentration with time became lower with the increase in radiation dose. The decay of polymer radicals formed upon irradiation with ␥-rays in N 2 gas and stored in N 2 gas was slow except for UHMWPE irradiated to 25 KGy. Irradiation with electron beams in air and the subsequent storage in air yielded a high concentration of polymer radicals, but the decay was much faster than that of ␥-irradiated UHMWPE. The g-factor evaluated from the ESR spectra revealed that the polymer radicals transformed into oxidized ones during storage in air. The UHMWPE radicals formed by ␥-irradiation in N 2 gas to 1-5 MGy remained almost unchanged during storage in N 2 gas. The gel fraction and the carbonyl and double bonds concentration increased with the increase in radiation dose when UHMWPE was ␥-irradiated in air, while the melting temperature of UHMWPE increased upon irradiation in air, so far as a UHMWPE block, not powder, was ␥-irradiated. The creep deformation decreased and surface hardness increased by ␥-irradiation in air, suggesting that crosslinking was introduced into UHMWPE molecules upon irradiation to high doses. In conclusion, irradiation of UHMWPE with ␥-rays and electron beams to high doses could introduce crosslinking to UHMWPE, resulting in increase in surface hardness and decrease in creep deformation. The trapped radicals in the irradiated UHMWPE decreased more quickly for electron-beam irradiation than for ␥-ray irradiation.