A model has been developed for the calculation of the microstructural evolution in a rapidly directionally solidified immiscible alloy. Numerical solutions have been performed for Al-Pb immiscible alloys. The results demonstrate that at a higher solidification velocity a constitutional supercooling region appears in front of the solid/liquid interface and the liquid-liquid decomposition takes place in this region. A higher solidification velocity leads to a higher nucleation rate and, therefore, a higher number density of the minority phase droplets. As a result, the average radius of droplets in the melt at the solid/liquid interface decreases with the solidification velocity.