Experiment was performed to investigate the applicability of energy condition for unstable fracturing as a mechanism of the fracture transition in Tipper specimen. Distributions of plastic work were estimated through hardness survey near fibrous crack. As the result it was revealed that the amount of plastic surface work in fibrous fracture was exceedingly large and widely different from the one in cleavage fracture. Accordingly any energy condition for unstable fracturing was expected never to be satisfied, in so far as the type of fracture remained fibrous. Further experiment was made about another unstable condition for ductile fracturing proposed by Orowan. It was confirmed that his theory represented rightly the unstable fracturing, though Tipper specimen was far from this condition in case where its length of fibrous crack grew to the order of critical crack length, C_cr . A new stress condition was proposed to illustrate schematically the mechanism of fracture transition in Tipper specimen as shown in Fig. 17, in which the behaviors of fracture stress curves were according to the results of recent study on structural mild steel.