The dynamic fracture toughness of Cr and Mo steels with nickel contents of 0, 1.56, and 5.5 wt.% was evaluated and compared to a lightweight steel of composition Fe-30.40%Mn-8.83%Al-l.07%Si-0.90%C-0.53%Mo. Each steel was heat treated to a Rockwell C-scale hardness range of 36 to 38. The 4130, 4325, and HY130 steels were quench-hardened and tempered. The lightweight steel was solution treated, water quenched, and age hardened. In the hardness range of Rockwell C36 to C38, the lightweight steel, the 4325 steel, and the Al-killed and Ca-treated HYI30 steel had similar dynamic fracture toughness values of 153,153, and 165 kJInr, respectively. The 4130 steel had a much lower toughness of 94 kJ/m2. The lightweight Fe-Mn-Al-C alloy performed better at Rockwell C32, producing the highest dynamic fracture toughness of 376 kJ/m 2. Toughness of the Cr and Mo steels was strongly dependent on deoxidation practice. Alloys treated with ferro-titanium showed a reduction in toughness, which was attributed to TiN particles and in one case eutectic Type II sulfides. Addition of misch metal to an aluminum and ferro-titanium treated HY130 steel eliminated the Type II sulfides and increased the dynamic fracture toughness from 58 to 88 kJInr. HY130 obtained the highest toughness (165 LJ/m2) when aluminum deoxidation was followed by calcium treatment.