A theoretical model is suggested which describes the generation of nanoscale grains due to stress-driven splitting and migration of grain boundaries in nanomaterials and Gum metals. In the framework of the suggested model, the stress-driven splitting and migration of grain boundaries are described as those related to transformations of grain boundary disclinations. The energy and stress characteristics for generation of nanoscale grains through stress-driven splitting and migration of grain boundaries in nanocrystalline aluminium, nanocrystalline nickel and Gum metal with the Ti-Nb-Ta-Zr-O composition are calculated.