Metal organic decomposition (MOD) Er2O3 coating for tritium permeation barrier was fabricated on two ferritic steels with dip-coating method. The interfacial layers, which were formed by the oxidation of the substrates, were found under the coating with different compositions and thickness according to the elemental depth profile of XPS. Their formations depended on the substrate materials (JLF-1: Fe-9Cr-2W based reduced activation ferritic/martensitic steel; SUS430: 18Cr based commercial ferritic steel) and the baking atmosphere (air or Ar). The main reason could be selective oxidation of main elements in the substrates at high temperature with the different baking atmosphere. For the coated JLF-1 samples, the surface smoothness and the hydrogen barrier performance of Er2O3 coatings were improved significantly by changing the baking atmosphere from air to Ar. The composition change in the oxidized interfacial layer from iron oxide to chromium oxide may be the reason for the improved surface smoothness and permeation barrier performance.