High-resolution solution ,13,C NMR spectra on radiation cross-linking in high density polyethylene by ,60,Co γ-rays has been observed. Two of the cross-linking new structural units identified in the present study for irradiated polyethylene are H- and Y-type, which formed during irradiation of polyethylene in vacuum both in the solid state and in the molten state.,Other radiation induced structural changes in the samples have been also detected. It is show that the concentration of Y-type formed during irradiation in vacuum in the solid state is greater than that of H-type, while H-type formed during irradiation in vacuum in the molten state is greater than Y-type. The results of this study have led to new insights into mechanism of radiation induced linking of polyethylene, and provide means of characterization of irradiated polymer.