The effect of hydrogen gas environment (external hydrogen) and hydrogen-charging (internal hydrogen) on the fatigue crack growth (FCG) in two materials, austenitic stainless steel Type 304 and ductile cast iron, was investigated at various test frequencies. The pressure of hydrogen gas was 0.7 MPa. Both in the tests of external hydrogen and internal hydrogen, ratio of hydrogen-induced FCG acceleration was gradually increased with a decrease in test frequency in the range of 10 ∼ 0.1 Hz, and then peaked out at 0.1 ∼ 0.01 Hz. The frequency at the maximum acceleration was dependent on materials and test types (i.e. external hydrogen or internal hydrogen). It has been pointed out that, in the test of external hydrogen, a small amount of oxygen impurity contained in hydrogen gas, if any, adsorbs on newly-created crack surface, which inhibits hydrogen penetration into the material near crack tip. Lower frequency allows longer time for oxygen adsorption, and consequently, hydrogen-induced acceleration cannot be prominent at very low frequencies (e.g. 0.001 Hz). However, in this study, similar frequency dependence of hydrogen-induced FCG acceleration was also observed in the case of internal hydrogen. The results inferred the presence of another mechanism producing the frequency dependence of hydrogen-induced FCG acceleration, i.e. hydrogen-induced slip localization dominated by the gradient of hydrogen concentration ahead of crack tip.