The microstructures and mechanical properties of two new aerospace Al-6.00 Mg-0.40Mn-0.12Zr (wt%) alloy sheets, containing 0.10 wt% Sc (low content) and 0.25 wt% Er(cheap), respectively, were investigated by tensile tests and electron microscopy methods. The results showed that microalloying elements were present in the form of core-shell-structured secondary Al3(Sc1−xZrx) and Al3(Er1−xZrx) nanoparticles, whose cores were enriched in Sc and Er, respectively. Stable core-shell structured nanoparticles enabled the annealed sheets to retain a completely non-recrystallized structure and strong β-fiber rolling textures. The ultimate tensile strength (UTS), yield strength (YS) and elongation to failure (Elf) of the annealed sheets reached 422 ± 1 MPa, 312 ± 6 MPa, and 20.7 ± 1.5% in the Al-Mg-Mn-Sc-Zr alloy, and 404 ± 5 MPa, 283 ± 7 MPa, and 24.2 ± 0.9% in the Al-Mg-Mn-Er-Zr alloy, respectively, both exhibiting high strength and superior ductility. The mean diameters of the spheroidal Al3(Sc1−xZrx) and Al3(Er1−xZrx) particles were 12.1 ± 4.2 nm and 20.2 ± 8.4 nm, respectively, meantime, the number densities of the Al3(Sc1−xZrx) and Al3(Er1−xZrx) precipitates were (7.7 ± 3.2) × 1013 m2 and (6.4 ± 1.9) × 1012 m2. The higher number density and the smaller particle sizes of the Al3(Sc1−xZrx) leaded to the higher strength of the Al-Mg-Mn-Sc-Zr alloy. The main strengthening mechanisms from the secondary Al3(Sc1−xZrx)/Al3(Er1−xZrx) nanoparticles were associated with the direct Orowan precipitation strengthening and sub-structure strengthening. Based on the results of this paper, enhanced mechanical properties and a reduced cost can be simultaneously achieved with the new Al-Mg-Mn-Zr alloys with low Sc contents or inexpensive Er addition, offering great potential for the development of new high-strength micro-alloyed AlMg alloys in industrial applications. Display omitted •Successfully prepare two new Al-Mg-Mn alloys with secondary Al3(Sc1−xZrx) and Al3(Er1−xZrx) nanoparticles•Characterize the microstructure evolution of new alloy sheets during their preparations•Clarify the existing forms of the combined additions of low-content Sc and Zr or Er and Zr•Discuss the potential for developing new cost competitive Al-Mg-Mn-Sc-Zr/ Al-Mg-Mn-Er-Zr alloys