In order to clarify an influence of hydrogen on the fatigue crack propagation in ultra-low frequency region, we investigated the crack propagation rates of S10C at different frequencies in hydrogen and nitrogen atmospheres. In the low-pressure hydrogen gas atmosphere, the crack propagation rate decreased with decreasing in frequency within the present experimental range. In particular, the crack propagation rate at the ultra-low frequency (10⁻³ Hz) became the same as that in nitrogen atmosphere. To explain the disappearance of the hydrogen effect in the ultra-low frequency, we proposed that carbon diffusion causing strain-age hardening also contributes to the decrease in crack propagation rate in the ultra-low frequency under the hydrogen atmosphere.