Abstract: Crack initiation and propagation in tempered carbon steels during the course of fracture due to repeated impact have been investigated by direct microscopic observation of fractured surfaces. Results of investigation have shown that, for the materials tested and impact energy level used, the depth of crack (t) is related to the number of blows (N) by an exponential function in the form t＝t0e（N/b） while the rate of crack propagation (V) has a straight line relation with its depth (t) given t＝t0e（N/b）It has also been shown that the total endurance life is governd both by the rate of crack propagation (V) and the final depth of ,crack (tx). Their effects on the total endurance life vary with the tempering temperature or with the carbon content in steel in the opposite ways indicating that the rate of crack propagat is primarily related to the strength factor while the final depth of crack is primarily related to the ductility or toughness factor. An optimum combination of these two factors results in the appearance of a peak value of total endurance life (resistance against repeated impact)at a certain tempering temperature or a given carbon coatent raoe results strongly supports our view points and general relationships concerning the resistance against repeated impact of tempered carbon steels postulated in papers [3][4][5],while in turn have laid new grounds for the development of strength potentiality of steels.