A significant size effect is found in the Al3Sc dispersoid-mediated precipitation in an Al-Mg-Si-Sc alloy.When the Al3Sc dispersoid size smaller than about 40 nm,β“ precipitates nucleate directly on the coher-ent dispersoids and grow by sacrificing the latter.While the dispersoid size greater than ～ 40 nm,Q\'and U2 phases are additionally produced that nucleate on the dislocations induced by the semi-/incoherent dispersoids.Mechanical and electrical properties are highly sensitive to the Al3Sc dispersoid-tuned pre-cipitation.The co-precipitation of β”,Q\'and U2 phases leads to an obvious improvement in hardness and simultaneously in electrical conductivity.