Photo-oxidation of polyethylene (PE) has been studied thoroughly these years owing to its high output and various applications, which makes knowledge about its degradation of vital practical importance. Polyethylene products often suffer from service environment factors including ultraviolet, heat or chemicals, which may deteriorate their mechanical properties. The degradation behavior varies with the chain structure and morphology of polyethylene, e.g., branches, linear segments length and crystallization. Natural degradation of three kinds of polyethylene (HDPE, LDPE and LLDPE) has been examined in samples using different annealing conditions in this work. The extent of degradation, described as carbonyl index, was evaluated by Fourier transform infrared spectroscopy (FTIR). Crystallinity measurements were made using differential scanning calorimetry(DSC). The investigation indicates that the branch structure plays a dominant role in the photo-oxidation of polyethylene. Annealing will change the crystallinity of polyethylene before aging, but do not show obvious dependence on the oxidation.