Ion-implantation on high purity iron substrates with nitrogen ions were carried out by using a Cockcroft Walton type accelerator under an accelerating voltage of 150 kV. Hardness measurements on the implanted surfaces showed that hardness effectively increased in the cold rolled specimens by ion-implantation in comparison with in an annealed specimen. Iron nitride, Fe16N2, was formed in the ion-implanted specimens. In the annealed specimen, relatively large particles of Fe16N2 were formed with low number density, while in the deformed specimens, dislocation substructures due to cold rolling were disappeared by ion-implantation and fine particles were densely formed. Strain field around dislocations induced by deformation provides nucleation sites for Fe16N2 particles.