Abstract: The deformation process and the strain field ahead of crack tip in Fe-3% Si single crystal were studied with SEM, grid method and computer image processing The distribution of strain field in plastic band around the crack tip is characterized by εyy= f(x)exp(-Ar) The general feature of the plastic deformation is similar in both charged or uncharged hydrogen specimens. However, the presence of hydrogen enhances the localized plastic band inhomogeneity and significantly increases the plasticity in high deep s...