The effect of oxides as an agent of ferrite solidification of iron and steel was investigated by measuring melt undercooling for solidification on single-crystal oxide substrates. Four kinds of single-crystal oxide plates, Al₂O₃(0001), Al₂O₃(11-20), MgO(100), MgO(111), were employed. Poly-crystal oxide substrates of Al₂O₃, MgO, Ti₂O₃ and V₂O₃ were also used for comparison. Undercooling for solidification of Ti-free C–Mn steel was dependent on the kind of oxide used, and seemed to increase with increasing the lattice misfit between the oxide and delta-ferrite, the primary phase to solidify. On the other hand, undercooling for the solidification of 1% Ti-added C–Mn steel was not so affected by the kind of oxide, and was lower than that of Ti-free steel solidified on the same oxide substrate. Titanium oxide or titanium-enriched oxide was formed at the interface between Ti-added steel and oxide substrates used, and this is expected to be the reason of decrease in undercooling. The undercooling of Ti-free steel on poly-crystal Ti₂O₃ substrate also supported this. Steel-oxide interfacial energy was estimated based on the undercooling measurements, and the result suggests that the interfacial energy between delta-ferrite and titanium oxide should be much smaller than that between the ferrite and other oxides, being close to metal–metal interfacial energy rather than metal-oxide interfacial energy. This metallic characteristic of titanium oxide is thought to be the primary cause to enhance the ferrite nucleation at the interface.