Abstract: The microstructure and mechanical properties of Ti-microalloyed and Nb-microalloyed Q345C steels were studied contrastively.The results show that the two kinds of steels both had high strength,good extensibility and excellent low-temperature impact toughness.Ti-microalloyed Q345C steel had better extensibility and low-temperature impact toughness,but it had lower strength.The microstrure of the Ti-microalloyed steel was similar to that of the Nb-microalloyed steel,both of them were composed of ferrite and pearlite and the grain sizes were 9-11 grade.For the Ti-microalloyed steel,the main inclusions were MnS and a few Ti4C2S2,the precipitation was TiN with sizes of 50-250 nm,and its strengthening mechanism was fine grain strengthening.For the Nb-microalloyed steel,the main inclusion was MnS,the precipitation was Nb(C,N) with sizes of 20-100 nm,and its strengthening mechanism was fine grain strengthening and precipitation strengthening.