In this study, stress intensity factor range (Delta K) decreasing tests were conducted and the in situ observations were used to investigate the fatigue crack growth behavior of JIS SCM440 steel near the fatigue threshold in a 9-MPa hydrogen gas, environment. The fatigue crack growth rate reflected the threshold behavior of the material, although the crack propagation knee point immediately before the threshold stress intensity factor range (Delta K-th) could not be distinctly identified. The fatigue crack was also observed to exhibit uneven propagation immediately before Delta K-th. In contrast, the knee points in a helium gas environment and air were very distinct. Fractographic analysis further revealed the existence of inter granular facets, which were observed immediately before Delta K-th in the hydrogen gas environment. Conversely, no facet was observed immediately before Delta K-th in the helium gas environment and air. The formation of the facets was considered to be one of the causes of the uneven crack propagation immediately before Delta K-th in the hydrogen gas environment. (C) 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.