A model is proposed which describes nanocrack generation and growth near the tips of blunt cracks in nanocrystalline metals and ceramics. Within the model, the stress concentration near the tips of large enough blunt cracks induces grain boundary sliding that leads to dislocation formation in triple junctions of grain boundaries. The stress field created by these dislocations and the applied load near crack tips initiates nanocrack generation and growth. It is shown that an increase in the curvature radius of blunt crack tips and a decrease in grain size promote nanocrack growth. These tendencies are in good agreement with experimental data concerning low values of fracture toughness and ductility of most nanocrystalline solids.