Abstract: The effects of microalloy elements such as Nb and Ti on the impact toughness of high strength structure steel Q690D were studied by using the impact testing machine, optical microscope and scanning electron microscope. The results showed that the impact energy of the steel was reduced due to the second phase particle coarsening and grain mixing, but there were such noticeable changes when Nb and Ti content of the steel were reduced respectively that the impact energy of the steel in the experiment could maintain above 90 J at -20 ℃.