Effects of hydrogen on the macroscopic behavior of crack growth in a thin sheet of single-crystal Fe-3wt%Si alloy were investigated. The center-cracked specimens were tested under a sustained load in a hydrogen environment while under continuous stretching in an air environment. A comparison between the macroscopic behavior of the specimens tested in hydrogen and air was made to elucidate the effects of hydrogen. The results show that hydrogen lowers the driving force for crack growth in the hydrogen environment. COTA is approximately constant in a hydrogen environment while it slightly increases in an air environment. Also, the necking region near the crack tip during crack growth increases linearly with the crack length in both environments; however, at the same crack length, a smaller necking region is observed in the hydrogen environment. Since the necking region can be regarded as the measure of the crack-tip plastic zone size, the necking region is speculated to increase linearly with the crack length, which is consistent with Dugdale's equation formulated for a perfect plastic body.