<p>MgO/Mg composites were prepared via in situ reaction of various contents of nano-sized ZnO (1, 4, 10 wt%) with Mg melt. Microstructural results showed that ZnO particles are completely reduced into MgO particles by Mg melt and obtained MgO/Mg composites. Typical MgO/Mg composites consist of α-Mg matrix and well dispersed MgO particles as well as dissolved Zn solute. In addition, micro-sized Mg-Zn intermetallic are also found when 10 wt% ZnO is introduced. Compared with commercial purity Mg, the ultimate strength, 0.2% yield strength, and the elongation of Mg—4 wt% ZnO are increased by 101, 143 and 74%, respectively. The strengthening mechanisms are mainly composed of grain refinement strengthening and composite strengthening induced by MgO particles. When ZnO content is below 4 wt%, the strengthening mechanism of MgO/Mg composites is predominated by the composite strengthening. As ZnO content is increased, the contribution of grain refinement is gradually beyond that of the composite strengthening, significantly improving both the yield strength and ductility. When ZnO content reaches 10 wt%, the elongation of MgO/Mg composites greatly decreases due to the agglomeration of MgO particles and the formation of Mg-Zn intermetallic.</p>