Monotectic alloys have great potentials to be used in industry due to their special physical and mechanical properties. But these alloys have an essential drawback that just the miscibility gap in the liquid state poses problem during solidification. When a homogeneous, single-phase liquid 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. Recent researches demonstrate that the only effective method to prevent the formation of the phase segregated microstructure in the immiscible alloys is using the rapid or sub-rapid solidification processing techniques. Laser surface treatment is a well known rapid solidification technique. It is widely applied in the industry to improve the surface properties of different type of alloys. But up to date, the solidification behaviors of the monotectic alloys under the laser surface treatment conditions have not been investigated. In this work, laser surface treatment experiments with Al-Pb alloys were carried out. A numerical model was developed to describe the microstructure evolution in the surface layer of Al-Pb alloys under the conditions of laser surface treatment. The model was applied to calculate the microstructure formation in the surface layer. The numerical results have a good agreement with the experimental ones. Both of the numerical and experimental results indicate that the microstructure of the laser treated surface layer is determined by the re-melting, composition homogenization and solidification of the alloy. Laser surface treatment can lead to the formation of an Al-Pb surface layer with well dispersed microstructure.