In order to clarify the effect of hydrogen on Mode II fatigue behavior, fatigue tests on a bearing steel (SAE52100, HV=748) were conducted. Cyclic torsion both with and without a static axial compressive stress was applied to hydrogen-precharged and uncharged specimens. For both loading cases, fatigue lives were shorter for the hydrogen-precharged specimens than for the uncharged specimens. Mode II and Mode I fatigue crack growth rates in the hydrogen-precharged specimens were higher than those in the uncharged specimens. The density of Mode II fatigue cracks near the end of life (N/N f =0.97) was greater in the hydrogen-precharged specimen than in the uncharged specimen. Microstructural changes and an increased density of slip bands were formed only in the hydrogen-precharged specimens. From these observations, it is hypothesized that hydrogen enhances slip deformation and localized slip bands thus increasing the density of cracks in the hydrogen-precharged specimens.