<p>The effect of severe plastic deformation by high-pressure torsion (HPT) on the formation of nanocrystalline structure in niobium has been studied by the methods of transmission electron microscopy, scanning tunneling microscopy, X-ray diffraction analysis and microhardness measurements. The thermal stability of the resulting structure has been analyzed. Specific features of grain boundaries (GBs) formed under the severe plastic deformation have been considered and analyzed. It is demonstrated that a considerable fraction of these boundaries are in the so-called nonequilibrium, or deformation-modified state, characterized by higher free energy and larger free volume in comparison with those of common relaxed GBs. It has been shown that contribution of nonequilibrium GBs to total strengthening is relatively small.</p> Graphical Abstract <p></p>