Abstract: Hydrogen-induced modification in the deformation and fracture of a precipitation-hardened Fe-Ni based austenitic alloy has been investigated in the present study by means of thermal hydrogen charging experiment,tensile tests,scanning electron microscopy (SEM) and transmission electron microscopy (TEM).It is found that the γ′ particles are subjected to the multiple shearing by dislocations during plastic deformation,which promotes the occurrence of the dislocation planar slip.Moreover,the alloy will be enhanced by hydrogen resulting in the formation of strain localization at macroscale.So,the mechanisms of deformation and fracture in the alloy have been proposed in terms of serious hydrogen-induced planar slip at microscale which can lead to macroscopic strain localization.