The microstructure evolution during the liquid-liquid phase transformation of Al-Pb alloy was calculated. The numerical results indicate that the interaction between the minority phase droplets has effect on the nucleation process of the droplets, and the effect increases with the cooling rate and the content of Pb.liquid-liquid phase transformation, nucleation, microstructure development Liquid-liquid phase transformation occurs when a homogeneous, single phase liquid is cooled into the miscibility gap in the liquid state. Generally the liquid-liquid phase transformation begins with the nucleation of the minority phase droplets. These droplets grow then by the diffusion of solute in the matrix. The nucleation and diffusional growth processes dominate the formation of the solidification microstructure of immiscible alloys under the rapid solidification conditions. A fine dispersion of particles can be obtained by using the fast cooling technique [1][2][3][4][5][6][7] . Recently great attention has been paid to the study of the solidification of immiscible alloys [8][9][10][11][12] . But commonly the microstructure evolution during a cooling was investigated by neglecting the interaction between the minority phase droplets. The objective of this paper is to uncover the effect of the interaction between the minority phase droplets on the dispersoid formation in a rapidly solidified immiscible alloy, especially on the nucleation behavior during the liquid-liquid phase transformation.