A modified casting Al–Cu alloy with ultrahigh tensile strength and ductility of about 520 MPa and 13.5% was obtained by PrxOy addition. PrxOy was decomposed to form AlPrO3, which acted as the effective heterogeneous nuclei for the crystallization of the primary α–Al phase. The main reason for the simultaneous increase in the strength and ductility of the modified alloy may be attributed to the effect of a large number of regular, network, and homogeneous nanoscale θ′ phase precipitates and more crystal grain and dendrite boundaries formed by their refinement on restricting and impeding the dislocation actuation and movement.