Ultra-fine grained steel bars were recently developed by thermo-mechanical controlled rolling with rapid cooling for increasing the strength of low carbon and low alloy steels. The developed steels are characterized by fine ferrite grains of less than 1 mum and high strength as a result of grain refinement. However, their correlations between tensile properties and impact behavior are not well understood. In this paper, impact absorbed energy (E-p) and dynamic fracture toughness (J(Id)) were used to evaluate the dynamic fracture behavior of the ultra-fine grained steels, and the fracture mechanisms were also investigated. For the ultra-fine grained steels, tensile stress-strain curve was shown to be correlated with the impact curve of load vs. time, and to be related to the dynamic fracture toughness. The steel with large ferrite grains, small ferrite grain colony and martensite was found to have a good combination of strength and toughness. (C) 2004 Kluwer Academic Publishers.