Dense polycrystalline wüstite samples equilibrated with 50%CO-CO₂ gas mixture for 432 ks were reduced at 1 073 K using 75%CO-CO₂ to study iron nucleation process. Especially the effects of grain boundaries and grain orientations on the iron nucleation were focused. The orientation of surface grains in the wüstite specimens was measured by applying electron backscattering pattern technique. The number of formed iron nuclei near grain boundaries was found to be significantly larger than that away from grain boundaries. The prevalence of the iron nucleation process near grain boundaries was mainly due to the fast transport process of holes and Fe divalent cation by surface and grain boundary diffusion. The dominant effect of the grain boundary on the iron nucleation was found on the all grains with different grain orientations.