Abstract: Influence of niobium content on the morphology of the martensite-austenite(MA) constituents in the coarse grained heat affected zone (CGHAZ) of weld and the toughness of CGHAZ was studied by simulating CGHAZ of five kinds of high strength steels with different niobium content.The results show that the volume fraction of the slender M-A constituents reduced with the decrease in niobuim content,thus the toughness of CGHAZ could be controled.As a result,niobium content should be reduced during the chemical composition design for high strength steel if the strength was satisfied.