Metal oxide has been the focus of research for decades. In this work, besides the trigonal-structure Al2O3 nanotubes and orthorhombic-structure Al2O3 nanosheets, novel Mg-Al-O/Al2O3 heterostructure nanowires were successfully synthesized at high temperature. The structural characteristics of these oxides were investigated based on electron microscopy and group theory analysis. More importantly, it was found that incorporating the orthorhombic-structure Al2O3 could effectively modulate the occurrence of monoclinic domains and thus the related properties of Mg-Al-O/Al2O3 heterostructures. This work provides a reference for understanding and modulating the complex structures with multiple domain variants in Mg-Al-O oxides.