Magnesium matrix composites were fabricated with the help of a reaction between magnesium and silicon oxide (SiO2) powders. The specimens were prepared by two processing routes (a powder processing route and an infiltration route). The basic reaction behavior was investigated by heating compacted SiO2 and magnesium powder blends. The effects of aluminum oxide (Al2O3) powder addition on the microstructures were investigated by the infiltration route. Magnesium oxide (MgO) and magnesium silicide (Mg2Si) were synthesized by the reaction between magnesium and SiO2. By the powder processing route, an explosive reaction took place when the volume blending ratios of SiO2/Mg were in between 20/80 and 30/70, and the specimens were highly porous. This explosive reaction became remarkable by decreasing the SiO2 powder size. The thermodynamic calculation revealed that the adiabatic temperature (Tad) of the specimen exceeded the boiling point of magnesium when SiO2/Mg were in between 20/80 and 30/70. Molten magnesium infiltrated into the SiO2 powder phase spontaneously under the heating condition of 800°C for 3.6 ks. The microstructure after the infiltration consisted of MgO, Mg2Si and magnesium. The course MgO agglomerates, which might be formed by the high temperature at aninfiltration front, were observed in the microstructure after the infiltration. By blending an Al2O3 powder (30 vol%, 50 vol%) in the SiO2 powder, the MgO phase became finer to several microns. The pores were hardly visible in the microstructure of the specimens fabricated by the infiltration route. The micro-Vickers hardness data showed that the hardness of the specimen was improved by blending the Al2O3 powder.