Microscopic features of crack growth in thin plates of single-crystalline Fe-3wt%Si alloy in both air and hydrogen environments were investigated by electron channeling contrast imaging (ECCI), electron backscattering diffraction (EBSD), as well as scanning electron microscopy fractography. The goal was to elucidate the discontinuous crack growth as well as the constant unit distance of crack extension (striation spacing). Center-cracked specimens were tested under a sustained load in a hydrogen environment, while they were under continuous stretching in the air environment. The following results were obtained. (1) Striation is formed by extensive slips emitted from the crack tip, mainly contributed from specific (11‾2)[1‾11] and (1‾12)[1‾11‾] slip systems. The discontinuous crack growth is mainly caused by interaction of the crack and (11‾2)[1‾11] and (1‾12)[1‾11‾] slip bands/cell walls formed ahead of the crack tip. These slip bands/cells show that the spacing between slip bands/cells is constant and independent of the crack length. Hence, the striation spacing is the same as that of the slip bands/cells ahead of the crack tip. (2) Hydrogen may affect the slip behavior by reducing the spacing between slip bands/cells ahead of the crack tip compared to that in the air environment.