In this paper a description for the phase formation in a binary planar diffusion couple is presented in the form of a system of differential equations. Not only diffusivity of constituents but also interfacial reaction at phase boundary are introduced in this system of equations. The formation process of phases including the sequence, the growth or shrinkage of the phases can be described clearly with this system of equations. The analysis on the basis of these equations indicates that the formation process of phases depends on interfacial reaction as well as of diffusion. Finally, two typical experimental results are expounded.