The mechanical properties and microstructures of Al–5.2Cu–0.4Mg–1.02Ag alloy during different aging processes were studied. The nucleation and coarsening of the main precipitation Ω phase were investigated, and the concentration ledge coarsening mechanism was proposed. The results show that the main precipitated phases are Ω phase and θ‘ phase A large number of fine Ω phase precipitates but a small amount of θ’ phase precipitates are found in the underaged alloy At peak–aging, the volume fraction of both Ω phase and θ‘ phase increases significantly and Ω phase issemi–coherent with the matrix. The equilibrium θ phase is found in subsequent overaging. During this time, Ω phase grows slowly but θ’ phase quickly both in length and thickness. Each of Mg/Ag co–clusters is used as a nucleation site of Ω phase in initial aging stage. The driving force for Ω phase coarsening comes from ledge migration caused by the atomic concentration difference of Mg, Ag and Cu. Since the segregation of Ag and Mg atoms in the interface between Ω phase and matrix reduces the misfit energy of lattice, the velocity of Cu atom moving to Ω phase is limited and Ω phase can keep its platelet shape well and the cohesion destabilization does not occur in long term aging.