Skew rolling is the process of reducing the diameter of a round billet. In this process, internal fracture may occur in the rolled material. This phenomenon is known as the Mannesmann effect. While there has been great discussion about the mechanism of internal fracture, it has yet to be fully clarified. To investigate the internal fracture initiation and propagation, we conducted hot rolling experiments and evaluated the stress and strain of the rolled material by elasto-plastic finite element analysis. The results show that the internal fracture arises on the internal surface in which shear stress acts owing to the combined effect of tensile and shear stress. In this paper, a new ductile fracture criterion is discussed to quantitatively predict the occurrence of internal fracture in skew rolling.