Abstract: Microstructure and impact toughness of CGHAZ in HSLA steels for oil tanks under high heat input (100 kJ/cm) have been investigated. Bainite is main microstructure in CGHAZ for four steels, but there is a significant difference in impact values due to different proportion of ferrite and granular bainite. Toughness values decrease with the increase of area percentage content of M-A constituents. The effects of morphology of M-A constituents on toughness have also been studied, and the harm of massive M-A constituent is more severe than that of long strip. Considering the influence of alloy elements on the formation of M-A constituents, area percentage contents of M-A constituents are predicted by the method of multiple linear regressions, which is helpful for evaluating the toughness of CGHAZ.