Small-angle X-ray scattering has been used to study the precipitation behavior of the δ′ phase in an Al-2.70mass%Li alloy treated by aging-retrogression-reaging. The results show that a transitional interfacial layer exists between the δ′ phase and the matrix in 8h reaging, but the transitional interfacial layer disappears in 16h reaging. This suggests that the δ′ phase particles are coarsened by “licking up” the transition interfacial layer before 16h reaging and swallow up each other after 16h reaging.