In the previous reports, it was revealed that the prestraining treatment at blue shortness temperature range was remarkably harmful to notch toughness of steel, evaluated by Charpy impact test, whereas this treatment was slightly detrimental to the so-called retained ductility, measured by round bar tensile test. To explain the contradictions between both effects of high temperature prestraining on retained ductility and notch toughness of steel thus treated, the authors investigated the conditions of fracture in the notched and unnotched specimens, combined with the varying effects of high temperature prestraining upon the cleavage and fibrous fracture stress curves, after scrutinizing the shapes of both curves of the steel prestrained by 10% at 300°C or 500°C by using notched bar tension specimen, suited notch shape of which for the present investigation had previously been searched and found out. In analysing test results, calculated distributions of plastic stress and strain in the notched bar tension specimen, as shown in the Appendix, were used. As the final results, it could be clarified that the remarkable fall of notch toughness of steel after prestraining at 300°C would chiefly be attributable to the elevation of the flow stress curve, as well as that of the fibrous fracture stress curve in the lower range of strain. It was also revealed that there would be no intimate correlation between the mechanism determining the notch toughness and that determining so called retained ductility.