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Effect of chemical compositions and microstructures on hydrogen embrittlement of austenitic stainless steel weld metals in high pressure hydrogen gas was surveyed by using the Slow Strain Rate Test. As a result, hydrogen emblittlement of weld metal was hardly influenced by delta ferrite in weld metal, but by stability of austenite phase, which was estimated by Md 30 value or Ni equivalent. In the weld metal with poor stability of austenite, ' martensite was formed near crack induced by SSRT. Additionally, though the crystal structure of ' martensite is as same as delta ferrite, susceptibility of hydrogen emblittlement became higher with the increase of ' martensite. The mechanism to explain the difference between delta ferrite and ' martensite was considered as following. The hardness, which increases the hydrogen embrittlement susceptibility in bcc structure, is higher in ' martensite than in delta ferrite. In addition, ' martensite might be formed continuously with propagation of crack. Therefore, the effect of ' martensite on hydrogen embrittlement could be larger compared with delta ferrite.