The effect of hydrogen gas environment on fretting fatigue strengths of pre-strained SUS316L and SUS304 was investigated. The tests were continued up to 3×10^7 cycles. Hydrogen gas pressure was 0.12MPa in absolute pressure. Fretting fatigue strengths of SUS316L and SUS304 in hydrogen gas were decreased than those in air. The reduction of fretting fatigue strength at 3×10^7 cycles in hydrogen gas environment compared with that in air was 7% for the pre-strained SUS316L and 24% for the pre-strained SUS304. Tangential force coefficient was higher in hydrogen gas than in air. The propagation of fretting fatigue crack started at 97% of the fretting fatigue life in hydrogen gas. Non-propagating cracks were observed in H_2 gas but not observed in air in the specimens of SUS304 endured 3×10^7 cycles of fatigue loading.
