In order to investigate the relationship between hydrogen and FCC/HCP phase stability in steel, slow strain rate tensile (SSRT) tests were conducted in Fe-30Mn-6Si and Fe-30Mn-3Si-3Al austenitic steels (the phase stability of the former steel is lower than the latter). SSRT tests were conducted in the following three cases: Case A) test in air, Case B) test in 0.7 MPa hydrogen gas and Case C) test in air using specimen charged with hydrogen at 270 ℃ and 10 MPa. In both Case B and C, Fe-30Mn-3Si-3Al steel did not show significant degradation of tensile properties by hydrogen compared with Case A. The Fe-30Mn-6Si did not show significant degradation of tensile properties by hydrogen in Case B but the unstable alloy showed a degradation of tensile properties by hydrogen in Case C.