Abstract: Two DH36 shipbuilding steels, containing 0.025wt% Nb and Nb-free, were produced by a controlled rolling and controlled cooling (TMCP) process, and then welded by 150 kJ·cm^-1 high heat input gas electric vertical welding. The microstructure and toughness of heat-affected zones of welded joints were studied. The results show that the addition of Nb could delay the transformation of ferrite and pearlite, and promote the formation of granular bainite and bainite ferrite, leading to less grain boundary ferrite and more granular bainite and bainite ferrite in the coarse-grained zone, and the slow precipitation of ferrite and pearlite in the fine-grained zone of the heat-affected zone of the Nb-bearing steel. The microstructure and toughness in different locations of the welded joints of Nb-free steel were good, and every single value of impact energy at -20 ℃ was above 102 J, which was far higher than the requirements of classification society. The Nb-bearing steel welded joint had single values of impact energy at -20 ℃ lower than 24 J, which did not meet the requirements of classification society, but every single value of impact energy at -20 ℃ in other locations was above 143 J. The addition of Nb was detrimental to the toughness of the welded joints of DH36 shipbuilding steel with high heat inputs.