Abstract: The limit strain and fracture from flat surface straight line flanging and under localized instability necking theory were compared to study the failure mode of aluminum alloy sheet in roller hemming process. The bending limit curve could be got by theoretical derivation based on ductile fracture, deformation theory of constitutive equation and combined hardening rule, and was verified by comparing with experiment result. Finally, the available range of ductile fracture criterion in roller hemming process was studied by numerical analysis. The results show that the bending limit curve based on ductile fracture could be adapted for prediction of roller hemming fracture of aluminum alloy sheets. Since the bending enhancement effect can′t be neglected in roller hemming, the localized instability necking theory for stretch bending can′t be applied to roller hemming process.