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The strengthening of micro-sized phases and nano-precipitates to the Al–Mg2Si–Cu(–Ni) alloys was studied at 25 °C and 350 °C. In the Al–15%Mg2Si–5%Cu alloy, lots of nano-precipitates (θ″, θ′, σ and Q′ precursor) are formed with the dissolution of Cu-rich phases after heat treatment. The high number density of nano-precipitates contributes to the high strength of alloy (357 MPa) at 25 °C. For the Al–15%Mg2Si–5%Cu–1%Ni alloy, more micro-sized Cu/Ni-rich phases are retained and fewer nano-precipitates (θ″, θ′, σ and S phase) are formed after heat treatment. The existence of high-temperature stable micro-sized Cu/Ni-rich phases produce an enhanced strengthening effect (109 MPa) to the alloy at 350 °C, compared to that of Al–15%Mg2Si–5%Cu alloy (82 MPa).