Abstract: The instrumented Charpy impact tests, the COD tests and the measurement of yield strengths were carried out on C-Mn steels, Cr-Mo steel and three kinds of weld metals under various test temperatures. It was found through the analysis of the test results that there is a unified correlation between the ductile-brittle transition temperatures of impact toughness and the COD for the low and medium strength steels and their weld metals. This correlation is a linear relationship between the transition temperature shift of the bi-toughnesses and the strain rate sensitivities of the test materials. On the basis of this relationship, the method to estimate the COD according to the impact toughness is proposed for engineering application. The internal cause of this correlation is considered as that the ductile fracture processes and the mechanical condition of cleavage during the V-notched impact are similar to those during the precracked slow bending, no matter what type of specimen is used.