Abstract: Positron annihilation lifetime spectroscope(PALS) was applied to investigate the micro-structural changes of polyethylene(PE) which was irradiated by γ-ray or ultraviolet radiation, the spectra were decomposed into three lifetime components using PATFIT package. Then it was found that the shifts of the degree of cross linking and crystallinity were detected effectively in samples. Moreover, a small quantity of oxygen, which involved in the crosslinking process, was measured sensitively by PALS. The regularity of positron lifetime intensity vs radiation intensity in γ irradiated sample was opposite to that in ultraviolet irradiated one, which is due to the preparation methods of samples and the change of polar functional group in initiator. Furthermore, contrastive patterns were studied by positron doppler broaden(DB) method and the results were agreed well with PALS data mentioned above.