When a homogeneous, single-phase liquid of monotectic alloy is cooled into the miscibility gap, the components are no longer miscible and two liquid phases develop. Generally, the liquid-liquid decomposition causes the formation of the microstructure with serious phase segregation. Many efforts have been made to use the demixing phenomenon for the production of well dispersed composite materials. It has been demonstrated that the rapid directional solidification technique is an effective method to prevent the formation of the phase segregated microstructure in immiscible alloys. Directional solidification experiments were carried out to study the influence of the addition of Sn on the solidification process of Al-Pb alloys. The experimental results show that the addition of a small amount of Sn causes a decrease in the interface energy between the matrix and the minority phase liquids and, thus, an increase in the nucleation rate of the minority phase droplets during the liquid-liquid phase transformation or a decrease in the average size of the minority phase particles. With the increase of the Sn content, both the volume fraction of the minority phase droplets and the temperature range of the liquid-liquid phase zone and the liquid-liquid-solid tertiary phase zone of the phase diagram increase. These are favorable for the coarsening of the minority phase droplets. The addition of Sn leads to the formation of a dendrite solid/liquid interface. This may promote the formation of a well dispersed microstructure and shows great effect on the distribution of the minority phase particles.