Abstract: This study investigates the effects of heat treatment on the morphology of long-period stacking ordered phase (LPSO) and the corresponding mechanical properties of Mg–9Gd–xEr–1.6Zn–0.6Zr (1–4wt%) using optical microscopy, scanning electron microscopy, energy-dispersive X-ray spectrometry, X-ray diffraction, transmission electron microscopy, and tensile tests. The microstructure analysis of the as cast alloys showed that the volume fraction of the (Mg, Zn)3(Gd, Er) phase increased with increasing Er content. The (Mg, Zn)3(Gd, Er) phase in grain boundaries transformed into the 14H-LPSO phase Mg12Zn(Gd, Er) after heat treatment between 400°C and 515°C. The bulk shaped LPSO phases distributed along the grain boundaries produced isometric crystal grains in and enhanced the ductility of the as-extruded alloys. However, the lamellar LPSO phases precipitating in the grain interior induced strip-shaped grains in and improved the strength of the as-extruded alloys. [Copyright &y& Elsevier]