Strong size effect on deformation twin-mediated plasticity in body-centered-cubic iron

•The atomistic plastic deformation mechanisms in nano-sized Fe are revealed by combining both the in-situ transmission electron microscopy and molecular dynamics simulations.•Fe NWs show three distinct size regions, in which the full dislocation slip (d < 2.5 nm), twin variant reorientation (2.5 nm < d < 17 nm), and TB cracking (d > 17 nm) dominate plastic deformation, respectively.•The sample dimension can be utilized to construct the Fe interconnections in nano/micro-devices with specific deformability.