In the Fe-Fe 2 Nb eutectic system, halo formations were observed at both the hypo-and hypereutectic compositions. This observation cannot be well explained by the existing halo theories based on competitive growth or competitive nucleating alone. In the present paper, a new explanation is proposed to describe the formation of halos in the binary Fe-Fe 2 Nb off-eutectic alloys. By theo retical calculations, the α-Fe and Fe 2 Nb are predicted to be the primary growing phase in the hypo-and hypereutectic compositions, respectively. Based on the experimental observations, the Fe 2 Nb Laves phase was identified as a good nueleant for α-Fe phase, but not vice versa. By analysing of the competitive growth among the involved phases and the growth undercoolings of the two individual eutectic phases, the formation of halos in the Fe-Fe 2 Nb alloys is explained successfully.