Three groups of cylinder specimens (φ10 mm×10 mm) with 20%, 30% and 40% (by mass, the same hereinafter) of ZrO2, were prepared using analytically pure MgO, Al2O3 and ZrO2 powders as the starting materials. The specimens were fired at 1 600 ℃ for 5 h, 1 700 ℃ for 0 h and 1 800 ℃ for 7 h, respectively. The microstructure of the MgO-MgAl2O4-ZrO2 subsystem was studied by FESEM. The results show that (1) in the temperature range of 1 600-1 700 ℃, solid phase reactions happen in the MgO-MgAl2O4-ZrO2 subsystem, leaving three phases: periclase, spinel and ZrO2 (solid dissolving with Mg2+) co-existing; spinellization affects the densification; (2) the specimen with 40% ZrO2 fired at 1 800 ℃ has heterogeneous melting microstructure; some areas form typical ternary eutectic MgO-MgAl2O4-ZrO2 with composition ofMgO 35.2%-37.2%, Al2O3 24.2%-26.6%, and ZrO2 37.2%-39.8%.