The corrosion-resistive intermetallic compound Fe3Si can non-electrolytically coat the iron substrate at 973-1173K in the molten salt composed of NaCl-KCl-NaF-Na2SiF6-SiO2. However, the adhesion of Fe3Si layer was weak because of the crack formation in the Fe3Si layer. The mechanism of the layer growth and defect formation was stud-ied experimentally. The marker experiments showed that the salt was caught up on the initial surface and left as the large voids in the layer. The fast deposition of silicon on pure iron surface and the quick growth of Fe3Si wrapped the salt in the layer at the first stage of deposition. The Si penetration from the Fe3Si layer into the a-Fe sub-strate was small due to the slow diffusion of Si in a-Fe. The fast Fe diffusion in the Fe3Si layer left the Kirkendall holes. The formation of these defects could be sup-pressed in the high Si concentration-iron alloys.