In recent years,materials with ultrafine grain size(UFG) have attracted much attention.By using severe plastic deformation(SPD) techniques,materials with fine grain size as small as 200-250 nm have been obtained.However,the nature of the grain boundaries has not been theoretically understood.It is still an unsolved question whether or not finer grain sizes down to 100 nm could be reached.A semi-quantitative model for the evolution of dislocation cells in plastic deformation was proposed.The linear stability analysis of this model leads to some interesting results,which facilitate the understanding of the formation of cell structures and of the factors determining the lower limit of the cell size of SPD materials.