The solidification behavior of an atomized droplet of a metastable, immiscible Cu–Fe alloy has been investigated experimentally and by numerical simulation. A model is presented to analyze quantitatively the microstructural evolution during the metastable liquid–liquid phase transformation. The model takes into account not only the common actions of nucleation, diffusional growth, and spatial motion of the minority phase spheres, but also the repulsive interaction between the spheres and the solid/liquid interface. The effect of the interface morphology on the collision and coagulation is first discussed. The formation mechanism of an Fe-poor layer and the kinetics of the metastable phase transformation in the undercooled liquid Cu–Fe alloy are sufficiently clarified.