Special aspects of nucleation and development of plasticity in nanocrystalline BCC-iron during shear are studied. The mechanisms of plastic deformation playing the key role in structural rearrangements during loading are revealed. It is shown that the development of plasticity can be conventionally divided into several stages. The first stage of plasticity evolution is associated with the formation and propagation of dislocations and twins. In the second stage, the main contribution comes from intragranular slip and grain boundary sliding. These processes initiate the grain shape changes. In the case of large shear values, the deformation behavior of the specimen is determined by the migration of interfaces. There is migration of both grain and twin boundaries. As a result of migration processes the grain size of nanocrystalline specimens is enlarged.