In order to investigate the effects of hydrogen on the fatigue crack growth behavior of low carbon steel JIS S10C, bending fatigue tests were carried out using a specimen with a small blind artificial hole in a low pressure pure hydrogen gas atmosphere. The results show that the fatigue crack growth rate in hydrogen gas is higher than that in nitrogen gas, moreover, the degree of acceleration is greater in the high strain range. In fractography, intergranular facets mixed with ductile fracture and quasi-cleavage fracture with brittle striations appear in a hydrogen gas environment, while only ductile fracture mainly appears in nitrogen gas. In the low growth rate range, many intergranular facets are seen on the ductile fracture surface, and in the higher growth rate range, quasi-cleavage facets increase as the growth rate increases. The growth rate of a small crack in nitrogen gas can be expressed by dl/dN ∝ Δεp nl in the wide range of applied total strain range Δεt. The same type equation is also satisfied in hydrogen gas, but in the narrow range roughly from Δε t=0.25% to Δε=0.37%. The fracture surface in this range shows only intergranular facets and a ductile morphology, but no quasi-cleavage fracture. Although the crack growth mechanism in hydrogen is different from that in nitrogen, observation of the mechanism of intergranular facet formation shows a similarity to the mechanism in nitrogen in which the slip-off mechanism of crack growth is valid. The formation of intergranular facets is also closely related to the slip behavior influenced by hydrogen. This means that there exists a high possibility for the application of the small crack growth law inhydrogen to not only S10C, but also to other carbon steels in which the intergranular facet appears.