The fretting fatigue strength of SUS304 is more significantly reduced in hydrogen as compared that in air. One of the causes is adhesion between the contacting surfaces and formation of many small cracks. In the adhesion mimicked fatigue test, hydrogen participates in the crack initiation. This is another reason for the reduced fretting fatigue strength in hydrogen. The effect of oxygen addition to hydrogen was also investigated. An increase in the fretting fatigue strength was found in the oxygen-hydrogen mixture. The friction force coefficient is reduced in this mixture as compared to that in pure hydrogen. The critical maximum shear...