The formation of in-situ spinel in Al2O3-MgO-C refractories along with different antioxidants occurs at different atmospheres in surface and inner of refractories. Then the types and microstructures of established phases have become increasingly important in these refractories. In the present work, reactions on firing Al2O3-MgO samples with added graphite, Al, Si and SiC in air or coke bed atmosphere at 1300°C and 1600°C have been studied. The results showed that with the addition of graphite and Al especially at 1600°C in coke bed, high alumina-rich spinel (Mg0.866Al1.83O3.611) was increased and in the microstructure were appeared high amount porosity. In silicon and silicon carbide additive samples after firing at 1600°C in coke bed, sapphirine phase were revealed.