It is well known that the metallic materials show the environmental embrittlement phenomena which cause ruptures at short period when stress is applied in various corrosive environments. The hydrogen embrittlement accompanied with the cathodic reaction is also included in these phenomena. It is said that the austenitic stainless steel SUS304 employed in structural materials causes stress corrosion cracking, but hydrogen embrittlement may or may not occur.In this study, in order to examine the hydrogen embrittlement behavior of SUS304 stainless steel from the change of mechanical properties or the change of fracture surface morphology, slow strain rate tension tests were carried out under the environment of hydrogen evolution immediately after the specimens were held for various time periods at hydrogen evolution potentials on the cathodic polarization curve which was obtained in 3%NaCl solution. The main results obtained are summarized as follows.(1) The reduction in area, which indicates, usually a remarkable change in the embrittled materials, decreased continuously with increasing holding time in each setting potential. This tendency became more pronouned at lower setting potential of -2.0V (vs. S.C.E.), and an embrittlement phenomenon could be found. The same results were also obtained for the change of time to fracture.(2) The fracture surface morphology accompanied with the embrittlement was represented by the transgranular fracture. The region of transgranular fracture containing the cleavage fracture occupied a wide range of fracture surface when the specimen was held for longer time at lower setting potential.