The effect of hydrogen atmosphere on the fatigue crack propagation property of JIS SCM435 chromium-molybdenum steel is investigated. The magnitude of fatigue crack propagation rate, da/dN, is significantly larger in hydrogen atmosphere than in helium atmosphere. The fracture surface and the near-tip cross-section are observed in detail by SEM and TEM respectively. The microscopic observations suggest that the fatigue crack propagation in helium is transgranular and dominated by plastic process. On the other hand, the fatigue crack propagation in hydrogen is brittle cracking along interface of grain and/or sub-grain structures. The crack seems to advance by unstable growth accompanied frequently by branching or deviation, which eventually results in the relatively large growth rate.