For slow strain rate tensile (SSRT) test in hydrogen gas, the degradation in relative reduction in area (RRA) of 300-series austenitic stainless steels is mainly attributed to hydrogen-assisted surface crack growth (HASCG) accompanied by quasi-cleavages. To establish novel criteria for authorizing various austenitic stainless steels for use in high-pressure gaseous hydrogen, a mechanism of the HASCG should be elucidated. At first, this study performed SSRT tests on six types of austenitic stainless steels, Types 304, 316, 316L, 306 (hi-Ni), 304N2 and 304 (N), in high-pressure hydrogen gas and showed that the RRAs were successfully quantified in terms of a newly-proposed nickel-equivalent equation. Then, to elucidate the microscopic mechanism of the HASCG, elasto-plastic fracture toughness (JIC), fatigue crack growth (FCG) and fatigue life tests on Types 304, 316 and 316L were carried out in high-pressure hydrogen gas. The results demonstrated that the SSRT surface crack grew via the same mechanism as for the JIC and fatigue cracks, i.e., these cracks successively grow with a sharp shape under the loading process, due to local slip deformations near the crack tip by hydrogen. Detailed observations of SSRT surface cracks on Types 304 and 316L were also performed, exhibiting that the onset of the HASCG occurred at the true strain of 0.1 or larger in high-pressure hydrogen gas.