Effect of IHT on low-temperature toughness of welten-80P 20CrMnSi and 20CrMnSiNi2A has been investigated in this paper. The results show that the impact toughness of IHT is increased 1-3 times as high as that of the conventional Heat Treatmeat. The Fracture appearance transition teapeatwe (FATT) is decreased by at utmost 54℃. Therefore IHT is a measure of increasing low-temperature toughness. In this way it is possible to apply this technology to production.