[en] Variation in the toughness of 0.50% Mo steel and the presence of low-toughness materials resulting from this variation create problems as the steel which has been widely used for naphtha, kerosene, and light oil hydrotreating reactor vessels. The toughness behavior of 0.50% Mo steel was examined using specimens with various Mo content, C content, and different heat treatment. The effect of hydrogen attack was investigated as a factor in the loss of 0.50% Mo steel toughness. As described in this paper, the examination revealed of that: (1) The toughness of 0.50% Mo steel varies widely with changes in microstructure and in tempering temperature and, therefore, can be stabilized and improved by properly controlling those conditions. (2) A low carbon content produces high toughness. However, the carbon content must be determined taking trade-offs with strength into consideration. (3) It was found that the conditions if (1) increase resistance to hydrogen attack as well, though the deterioration in the toughness occurs due to hydrogen attack