Abstract: The toughness and susceptibility to hydrogen-induced cracking of the t_8/5=5s and 65s weld overheated zone (WOZ) of T-1, StE690 and Wel-ten80c HSLA steel of 8OOMPa grade tensile strength were studied. The microstructure of the WOZ, in which the retained austenite content was detected, was observed and compared by TEM. It was shown that the feature of interface carbide in the martensite structure and micro-constitution of island-like structure in the B₁ structure are the important factors affecting the properties of the WOZ. When the retained austenite is in the shape of membrane or strip, the larger its amount is, the better it reduces the susceptibility to hydrogen-iuduced cracking in the WOZ.