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Detwinning is a unique deformation mechanism of nanotwinned metals with twin lamellae thickness down to a few nanometers. In this work we investigate the impact of detwinning mechanism on the tensile ductility of twinned Cu nanowires containing high density of parallel twin boundaries by means of molecular dynamics simulations. Simulation results show that the fracture strain of twinned Cu nanowires has a strong dependence on twin boundary spacing, resulting from the competition between individual deformation modes. Particularly for the twinned Cu nanowires containing the thinnest twin lamellaes, the dominant detwinning mechanism leads to a significant reduction in the tensile ductility. It is found that detwinning originates from twin boundary migration, which is a result of the glide of lattice partial dislocations on the twin planes. This work advances our fundamental understanding of the twin boundary-related mechanical properties of twinned metallic nanowires. [ABSTRACT FROM AUTHOR]