The limit of dislocation density was investigated by means of mechanical milling (MM) treatment of an iron powder. Mechanical milling enabled an ultimate severe deformation of iron powder particles and dislocation density in the MM iron powder showed the clear saturation at around the value of 10(16)m(-2). On the other hand, the relation between hardness and dislocation density was examined in cold-rolled iron sheets, and the linear Bailey-Hirsch relationship; HV[GPa]=0.7+3x10(-8)rho(1/2) was obtained in the dislocation density region up to 3x10(15)m(-2). Extrapolation of the Bailey-Hirsch relationship indicated that the dislocation strengthening should be limited to about 3.7GPa in Vickers hardness which corresponds to about 1.1GPa in 0.2% proof stress.