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The precipitation sequence in Al-Cu-Li alloys is sensitively dependent on the Cu/Li ratio. In the low ratio Cu/Li alloys of 1–2.5, the δ'phases usually nucleate and grow from the θ'precipitates, forming δ'/θ'/δ'composite precipitates. In this work, we present a first-principle study on atomic structures and their relative stabilities of the growing δ'/θ'/δ'composite precipitates in Al-Cu-Li alloys. Based on the analysis of the interface formation energy, constituted interface and coherent strains energies, an “anti-phase 1/2 [110]” relationship for the opposite δ'has been proposed when the inward θ'has an odd number of Cu-layers. It may be achieved by translating one side of the δ'by 2/2 a along the [110] slip direction, which is an energetically most favorable path. By analyzing the bonding characteristics, both the “zigzag Al-Li combined with Cu” and the “zigzag Al-Al” interfacial terminals are found to control the interface structure …