The theoretical model is suggested which describes a new micromechanism of strengthening in the ultrafine-grained Al-Cu-Zr alloy subjected to severe plastic deformation. The departure point in this theoretical model is the assumption that the key role in the plastic deformation of high-pressure torsion processed ultrafine grained Al is played by extrinsic grain boundary dislocations (EGBDs) gliding along non-equilibrium grain boundaries and forming dislocation pile-ups at triple junctions of the grain boundaries. Within the model, nanoprecipitates of Al2Cu at grain boundaries act as obstacles for the slip of extrinsic grainboundary dislocations (EGBDs) that leads to a significant increase in the strength of the Al-Cu-Zr alloy. The plastic deformation occurs through the emission of lattice dislocations from the pile-up of EGBDs pressed to a triple junction of grain boundaries. It is shown that the division of gliding EGBDs into separate pile-ups by nanoprecipitates can provide substantial additional hardening of the alloy. The proposed model is in good quantitative agreement with available experimental results.