Abstract: This paper edeals with two sorts of high-strength steel, 10Ni5CrMoV and 12Ni2CrMoVA.The change of the amount of lath martensite in the overheated zone of joints and the effect of the own characteristics of lath martensile on toughness have been investigated. By means of metallography, fractograph analysis and TEM, the relation of toughness to the behaviour of the lath martensite, density of dislocatinn and degree of deposition of carbides has been studied. The paper provides a basis both in theory and in practice for improving the toughness of high-strength steel joints.