JIS SUS304 fretting fatigue test was done in high-purity hydrogen, an oxygen hydrogen mixture and humidified hydrogen. The fretting fatigue strength in hydrogen was drastically changed depending on the oxygen level. Based on the XPS (X-ray photoelectron spectroscopy) analysis of the fretted surface, it was found that the fretting removed the original protection layer of the stainless steel, however, the addition of water vapor or ppm-level of oxygen produced an oxide layer on the fretted surface during the fretting that surpassed the removal effect of the initial oxide layer by fretting. In fact, a strong adhesion between the contacting surfaces occurred and no fretting wear particles were observed in the high-purity hydrogen. On the other hand, oxidized fretting wear particles were found in the oxygen hydrogen mixture. Based on the geometry of contact used in this study, a severe concentration of the contact pressure arouse at the contact edge. This produces a compressive stress field in the specimen where the crack growth was suppressed. This stress concentration was relieved when fretting wear occurs. Therefore, the change in the fretting fatigue strength in hydrogen by the addition of oxygen is closely related to the change in the wear behavior. Copyright (C) 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.