Microstructure characterization and transition of undercooled Fe-7.5 at% Ni melt were investigated, adopting melt fluxing and cyclical overheating. For the samples subjected to low undercoolings, single-phase microstructure results, whereas many second-phase particles, which are deemed as the so-called dendrite core originating from the primary solidification of the metastable delta phase, were observed in the as-solidified samples after high undercoolings. Applying the classical nucleation theory, the competitive nucleation between delta and gamma phases was Studied, thus giving calculations consistent with the observed experiment results. In order to understand the forming mechanism of the dendrite core, a schematic diagram incorporating solidification and solid-state transformation was given. On this basis the microstructural transition of the metastable phase was declared. (c) 2005 Elsevier B.V. All rights reserved.