Abstract: The influences of B content and Nb, V micro-alloying on the impact toughness of low carbon boron steel were investigated by impact tests of low carbon boron steels with different boron contents and of Nb, V micro-alloyed low carbon boron steel. The results show that the B content of 0. 000 6% to 0. 0015% can improve the impact toughness of the steel, but the B content more than 0. 003 96 decreases the impact toughness. Nb, V micro-alloying with appropriate Al can increase the impact toughness of the steel, especially after heat treatment. It is disadvantageous that the Ti content is higher than 0.03% to the impact toughness of low carbon boron steel and micro-alloyed low carbon boron steel.