In order to find the major reason for the radiation discoloration of PE, the changes of yellow index (,YI,) of HDPE and LDPE under ,γ, irradiation in air or N,2, and after irradiation are measured using colorimeter. The growth and decay of alkyl free radical formed in HDPE and LDPE, and the change of the absorbance of conjugated double bonds formed in PE are measured using ESR and UV —Visible spectrometer. The ,YI, of HDPE is higher than that of LDPE at the same radiation dose. The ,YI, of PE irradiated in air is higher than that of PE irradi,¬, ated in N2 at the same radiation dose. The ,YI, of PE irradiated different dose decreases with post—irradiation storage time firstly, up to a certain time, it becomes stable. It is found that the for,¬, mation and decay of free radicals is very similar to that of the change of ,YI, whereas,the change of the absorbance of conjugated double bonds formed in PE is different to the change of ,YI,. The results are discussed,it is believed that the major reason for the radiation discoloration is the for,¬, mation of trapped free radicals.