Oxidative degradation caused by γ-ray irradiation is a major problem that limits the durability of ultra high molecular weight polyethylene (UHMWPE) used for artificial knee joint systems. In this paper, we have investigated the oxidative degradation of UHMWPE surfaces in the depths of μm and nm by using infrared absorption measurement and by X-ray photoelectron spectroscopy (XPS), respectively. Infrared absorption shown by C=O peak indicates that the oxidation was uniform in the depth up to ∼200 μm. We found that the oxidation in the range of nm was appropriately evaluated by XPS of the O-1s spectrum region. It was also confirmed that the addition of vitamin E was effective to prevent the oxidation induced by γ-ray irradiation.