According to the microscopic observation, made by the author before, of the crack initiation of ferritic nodular cast iron, it has been proved that the fracture can be interpreted by considering the stress induced by the piled up dislocations at the end of a slip line near the graphite nodule. Founded upon this fact, the author, using Stroh's formula for the stress round a piled up group of dislocations at the end of a slip line, actually induces the formula for fracture strength under the action of the combined stresses. The above theory is then confirmed by the combined tension-torsin tests in liquid nitrogen on the thin hollowed cylindrical specimens.