A theoretical model is suggested which describes nucleation of nanoscale twins at locally distorted grain boundaries containing disclinations in nanocrystalline and ultrafinegrained materials. Within the model, nucleation of a nanotwin occurs through cooperative emission of partial dislocations from a grain boundary segment containing a disclination. We calculated the energy characteristics of the nanotwin nucleation process in question. Also, we revealed critical values of the external shear stress needed for realization of the nanotwin nucleation process in nanocrystalline and ultrafine-grained materials.