Abstract: Based on VOD vacuum refining process, the composition and morphology of MgO·Al2O3 compound inclusion in 409L stainless steel deoxidized by aluminum were analyzed. A phase diagram between MgO, MgO·Al2O3 and Al2O3 at 1873K was drawn through thermodynamic calculation to investigate the thermodynamic conditions of MgO·Al2O3 inclusion formation and transformation. The results showed that the pure MgO·Al2O3 phase of spinel compound inclusion in molten steel present amorphous structure, where the inclusion size is about 5μm. On the condition that Al content is controlled at 0.04 wt% in molten steel together with w(Mg) ≥ 1.3 × 10^-8, the MgO·Al2O3 inclusion will form Al2O3, it will transform into MgO inclusion if w(Mg) ≥9.7 × 10^-7.