Oxidative degradation of ultra-high molecular weight polyethylene (UHMWPE) attributed to sterilization by gamma-radiation in the presence of air has been cited as one of the major causes of premature wear in total joint arthroplasty. For example, in retrieved UHMWPE tibial bearings, not only adhesive and abrasive wear, but also fatigue wear characterized by delamination and fracture is frequently observed. In this study, we examined the effects of gamma radiation on the microstructural morphology of UHMWPE tibial bearings, and propose a severe fatigue wear mechanism. Scanning electron microscopic observations were conducted on freeze-fractured surface of retrieved UHMWPE components that had been sterilized with gamma radiation in air before implantation, unused components that had been stored on the shelf for several years (5-11) after sterilization, and disc specimens given an accelerated aging protocol after gamma radiation. Scanning electron microscopic observations showed that the freeze-fractured surface of these components had a double layer, which was bordered below the surface. A closer observation of the subsurface layer below the border revealed an extremely rough and porous honeycomb-like structure. Fourier transform infrared analysis demonstrated that the honeycomb-like region in the subsurface had a high oxidation level. The internal morphology of oxidized UHMWPE was classified into four categories based on the level of the oxidation. According to these results, the morphological changes with oxidative degradation of gamma-irradiated UHMWPEs in the presence of air could consistently be explained as the result of major chemical and physical changes such as increased crystallinity, elevated density, and reduced mechanical strength. We relate the morphological changes caused by oxidative degradation in the subsurface to the location of the origin of fatigue wear in total knee joints.