The effects of low-dose (1 to 5 Mrad) gamma irradiation in different gas environments on thermal and tensile properties, and mass of melt-crystallized ultrahigh molecular weight polyethylene (UHMW-PE) have been investigated. The gamma irradiation was conducted under four different environments, air, nitrogen, acetylene, and vacuum. Thermal parameters were investigated using differential scanning calorimetry. The results showed that both irradiation dose and environments affected the thermal and tensile properties. Gamma irradiation significantly increased the melting temperature of UHMW-PE in all cases. However, the oxidation temperatures of irradiated polyethylene was lowered in all cases except those associated with acetylene environment. The crystallinity of the polymer was increased upon irradiation. Tensile yield strength and modulus of irradiated UHMW-PE increased for all cases except those treated at 1.25 and 2.5 Mrad, and in nitrogen environment that showed a decrease in the modulus, Gamma irradiation caused a weight gain of UHMW-PE in all conditions. To further the study, the postirradiation aging effect on the irradiated polymer was examined during a period of 1 year, and the results showed that the thermal and tensile properties of UHMW-PE were, indeed, affected. The data suggests that irradiation in acetylene affects mainly the crosslinking of the polymer.