The grains in nanocrystalline copper have been found to grow at liquid nitrogen temperatures. See K. Zhang, J. R. Weertman and J. A. Eastman, Rapid Stress Driven Grain Coarsening in Nanocrystalline Cu at Ambient and Cryogenic Temperatures, Applied Physics Letters, 87, 061921 (2005). This unusual phenomenon has been analyzed using a dislocation model. It is found that purity (with many free dislocations in the grain boundary) and non-equilibrium structure (with many extra dislocations in the grain boundary ) are necessary conditions for mechanical grain growth without thermal activation. Computer simulation of the dislocation emission process from a grain boundary with extra dislocations and with approaching dislocations has been carried out. The dislocations appear to group together as short walls which move as a unit. It is found that highly stressed grain boundaries under an indenter will move to dissociate itself and the dislocations emitted from the dissociated boundary will approach nearby boundaries to dissociate them also at lower stresses. These processes start at the highly stressed region and continue to propagate outward to grow the grains by mechanical stress alone.