The objective of this study is the characterization of the fretting fatigue strength in a hydrogen gas environment. The test materials were a low alloy steel SCM435H, super alloy A286 and two kinds of austenitic stainless steels, SUS304 and SUS316L The test was performed in hydrogen gas at 0.12 MPa absolute pressure. The purity of the hydrogen gas was 99.9999%. The fretting fatigue limit was defined by the fretting fatigue strength at 30 million cycles. For all materials, the fretting fatigue strength in the hydrogen gas environment increased in the short-life region. However, the fretting fatigue strength in the hydrogen gas environment decreased in the long-life region when exceeding 10 million cycles except for SCM435H, while there was no reduction in the fretting fatigue strength in air between 10 and 30 million cycles. The reduction rate was 18% for A286, 24% for SUS304 and 7% for SUS316L The tangential force coefficient in the hydrogen gas environment increased when compared to that in air. It can be estimated that this increase is one of the causes of the reduced fretting fatigue strength found in a hydrogen gas environment. In order to discuss the extension of the fretting fatigue life in hydrogen gas observed at the stress level above the fretting fatigue limit in air, continuous measurement of the fretting fatigue crack propagation was performed in a hydrogen gas environment using the direct current potential drop method. As a result, it was found that the extension of the fretting fatigue life was caused by the delay in the start of the stable crack propagation. (C) 2009 Elsevier Ltd. All rights reserved.