The features of the plastic deformation at the atomic level and the formation of a dislocation structure in nanotwinned Hadfield steel under severe shear deformation along the direction perpendicular to parallel coherent twins were studied by the method of molecular dynamics. Similar studies for comparison were carried out for austenite. Dislocations in steel propagated and developed more weakly than in austenite, stacking faults between partial dislocations were shorter, and the number of dislocations was smaller, which was a consequence of the interaction of dislocations with impurity carbon atoms in steel. The main plastic shifts inside the computational cells containing parallel twins occurred predominantly along twin boundaries in the form of twinning dislocations. Dislocations from another slip system, not parallel to the twin boundaries, practically did not form. As a result of the passage of twinning dislocations, the twins moved during deformation.