Experiments were made to investigate the effect of prior strain history on the fracture stress curve and notch toughness of a killed mild steel. Various types of pre-straining were subjected to the steel plate, i.e., compressive straining by 10% or 20%, tensile straining by 10% in oblique direction, tensile straining by 5% after 10% compressive straining and straining at lower temperatures. Fracture stress curves were measured on those pre-strained steels. At the same time, standard V-notch impact tests were also performed. It was revealed after analysing the test results that the difference in strain history gave no appreciable effect on the fracture stress curves. Also it was known that the same correlation was found between the height of fracture stress curves and Tr₁₅ in those pre-strained steels, as reported for usual structural steels.