Gel fraction(%) of LDPE containing antioxidants and irradiated by fast electron beam(200 kGy) at ambient temperature and in air decreases by 6%—30% as compared with the control specimens. DSC measurement shows that the peak melting temperature T,m, enthalpy of fusion, peak crystallization temperature T,c, and enthappy of crystallization of the irradiated LDPE are somewhat declined relative to that of unirradiated specimens. The second heating and cooling DSC run demonstrates that the T,m, and T,c, of all the samples studied in this work are not changed, but their enthalpies of fusion and crystallization tend to decrease. The cooling DSC reveals no influence of radiation on the blocky distribution of the branches in the LDPE used in this work. The X-ray diffraction analysis for the heat-treated samples demonstrates that irradiation does not cause appreciable change of the crystal structure and crystallite size of the samples, but the degree of the crystallinity of the irradiated samples is somewhat lower than that of unirradiated ones.