A fracture toughness test under continuous cathodic charging was performed using four kinds of low alloy steels. The reduction in fracture toughness, J_IC, was characterized in terms of the effect by the material, hardness and loading rate. The reduction in J_IC was significant when a slower loading rate was used and a harder material was used. However, the 3.5NiCrMoV steel showed a relatively lower reduction in J_IC compared to the other materials even when the hardness was higher than that of the other materials. The fracture surface changed from dimpled to quasi-cleavage when the reduction in J_IC was significant.