The ultra-high molecular weight polyethylene/modified illite-smectite interstratified clay minerals composites（UHMWPE/M-ISIC） prepared by melt blending were irradiated by γ-rays with a dose of 75 k Gy in air and N2 atmosphere, respectively. The results showed that the tensile strength of UHMWPE/M-ISIC increased at first and then decreased with the increasing content of M-ISIC, and reached its maximum value within mass percent of 1% of M-ISIC. The thermal stability of UHMWPE after irradiation in air decreased significantly, while that of UHMWPE/M-ISIC decreased slightly. The thermal stabilities of neither material showed obvious changes after irradiation in N2 atmosphere. The crystallinity of UHMWPE after irradiation in air increased significantly, but decreased in N2 atmosphere, while that of UHMWPE/M-ISIC decreased after irradiation in both air and N2 atmosphere. The gel content and creep resistance of UHMWPE/M-ISIC after irradiation in air were higher than those of UHMWPE, and lower in N2 atmosphere. The results indicate that the M-ISIC irradiated in air could reduce the cracking by decreasing the diffusion rate of oxygen and thus promote the crosslinking effect. However, the crosslinking is hindered in N2 atmosphere as the free activity of the molecular chain is hindered.