The stress aging method was conducted for an Al-Cu-Mg-Ag polycrystalline alloy to study the competitive nucleation and growth behavior of GP zones, θ′ and Ω precipitates concurrently existing on different {001} and {111} habit planes. After initial stress aging for 1 h, the preferential nucleation of GP zones against Ω occurred irrespective of the stress direction applied to the grains. With increasing aging time, the density of Ω increased while the GP zones decreased in number to dissolve into the matrix, resulting from competitive nucleation due to the mutual constituent of Cu in GP zones and Ω. With further stress aging, the growth of θ′ plates was enhanced in comparison with the growth of Ω plates. The relative number density change between {001} and {111} precipitates was emphasized especially in grains with the stress direction along [001], which indicated that nucleation and growth of GP zones and θ′ plates were highly sensitive to external stress compared with that of Ω. HREM observation found that low amounts of small Ω plates were nucleated during the initial stress aging indicating retarded Ω nucleation. Two-step aging (stress aging for 1 h followed by the stress-free aging) revealed that preferentially nucleated GP zone against Ω during the initial stress aging determined the successive precipitation sequence of the precipitation of Ω and θ′.