Immiscible alloys are well suited as functional materials, such as bearings, electrical contacts,switches and superconductors, etc. They usually suffer from heavy segregation under ordinary casting, which is resulted from the decomposition within the miscibility gap of a homogeneous liquid into two immiscible liquids generally with distinct density difference. But this characteristic provides an opportunity to in situ fabricate composites with core-shell morphology. In this study, Al/Sn-Bi core-shelled particles have been successfully prepared by phase separation of Al₇₀Bi₁₁Sn₁₉ alloy. The morphology, microstructure, composition and phase transformation of the core-shelled particles were investigated by means of SEM, EDS and DSC. It reveals that the particle comprises an Al_-rich core with a Sn-Bi hypoeutectic shell, displaying a two-stage melting characteristic. The morphology of particles varies with size. With increasing the particle size from 0.5 mm to 0.9 mm, the core-shell morphology turns from a crescent multi-core type into concentric or eccentric single-core types. Based on the simulation of temperature field of Al₇₀Bi₁₁Sn₁₉ alloy droplets during solidification, the formation mechanism of the core-shell morphology has been discussed in detail, which is attributed to an outcome of the competition among the surface segregation, Marangoni and Stokes motions, Ostwald ripening and cooling rate.