Directional solidification of peritectic alloy described is significant for advanced materials processing. The nucleation and growth mechanisms of peritectic phase are outlined and the nucleation undercooling of peritectic phase is small as a result of the presence of properitectic phase nuclei. On the basis of assuming maximal interface growth temperature or nucleation and constitutional undercooling criterion (NCU), the microstructure and phase selection of peritectic alloy have been considered. It was found that the peritectic reaction as one of the growth mechanisms of peritectic phase existed not only at the S/L interface, but also in the directionally solidified sample of Sn-Cu peritectic alloy. Finally, the problems for directional solidification of peritectic alloys were presented to deeply and rigorously explore the solidification theory and experiment of peritectic alloy.