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Nanotwinned Cu exhibits an unusual combination of ultra-high strength and high tensile ductility. However, its fracture behavior and associated microscopic mechanisms remain largely unexplored. Here we study the fracture in a free-standing thin film of nanotwinned Cu using molecular dynamics (MD) simulations. For a pre-crack inclined to the twin boundary, MD simulations show a characteristic fracture mode of zigzag cracking, which arises due to periodic deflections of the crack path by twin boundaries. The mechanism of fracture involves the screw dislocation-mediated local thinning ahead of the crack, instead of cleavage fracture. Importantly, MD simulations show a unique fracture footprint of< 1 1 0>-oriented crack edges, consistent with the previous experimental observation from in situ transmission electron microscopy. Our results reveal the toughening mechanisms by nanotwins and also have broader …