A criterion of the ductile fracture of mild steels is investigated from the view point of mathematical theory of plasticity. Round bar test specimens are brought to fracture, some under simple tension, some others under simple twist and the rest prestrained under tension and then twisted. The true stress-strain curves at the very point of fracture initiation are carefully examined and correlations between fracture criteria for different stress states are investigated. As the conclusion the criterion is best expressed by the amount of slip energy on the fracture plane. To evaluate the amount of slip energy from a stress-strain curve it is neccessary to make use of the concept of slip theory of plasticity, which constructs plastic strains from integration of infinitesimal slips on every planes. The slip theory is suitable to evaluate the slip energy on any single plane from a stress-strain relation. On the other side another theories of plasticity are not appropriate to distinguish the slip energy on a particular plane from the total plastic strain energy which has no direct relation to the ductile fracture.