Mechanical milling treatment of iron powder was applied as the technique to charge an extremely large strain into iron matrix. Milling treatment using high energy vibration ball mill enable a significant work hardening of iron to Hv950 after a long time milling, whereas the maximum hardness achieved by conventional cold-rolling process for bulk iron is small as Hv280. For the mechanically milled iron powder with the hardness Hv950, the grain size is refined to several tens nm due to super heavy deformation through mechanical milling. On the annealing of this mechanically milled powder, it was confirmed that softening occurs not through the well-known recrystallization process but through gradual grain growth of such fine grains. The hardness of iron powders yields the Hall-Petch relation to the grain size, in the case they do not have substructures within each grain such as dislocation cells and subgrains. Mechanically milled iron powder can be consolidated to the bulk with full density at the low temperature of 923K and the grain size is kept at about 0.2mm due to the grain boundary pinning effect of finely dispersed oxide particles. 0.2% proof stress of this bulk iron is increased to about 1.6Gpa by grain refining strengthening. The behavior of grain refining strengthening is identical in both steels of fcc and bcc structure regardless of the crystal structure.