The solidification process in a vertical strip cast immiscible alloy has been analyzed and a model describing the microstructure evolution has been presented. The solidification of the strip cast Al-5Pb (mass fraction,%) alloy has been simulated by coupling the calculated temperature and concentration fields with the kinetic equation which controls the microstructure development. The numerical results show that there exists an undercooled region in front of the solid/liquid interface and the minority phase droplets nucleated there. The droplets grow by diffusion of solute in the supersaturated matrix as they move toward the solid/liquid interface. A higher solidification velocity leads to a higher supersaturation and, therefore, a higher nucleation rate of the minority phase droplets. Thus, the number density of the minority phase droplets increases and the average droplet radius decreases with increasing the solidification velocity.