Magnesium matrix composites reinforced with SiO2 coated carbon fibers have been investigated, with an emphasis given on the relation between the material strength and interfacial microstructure. The composites were studied as a function of aluminium (Al) content that is varied between 0 and 9 wt%. The obtained results indicate that the reactivity at the C/Mg-Al interface of the composite can be controlled by varying the Al content. The low Al content in C/Mg-1Al has been completely dissolved in the matrix with no segregation even after solidification, leading to the best mechanical performance. If the Al content is increased to >= 3 wt%(composites such as C/AZ31 and C/AZ91), the SiO2 coatings are fully depleted due to an extensive formation of carbides at the interface. The precipitates are further identified as Al2MgC2 phase that is similar to binary carbide Al4C3. SiO2 coating on the fiber layer prior to fabrication of composite is found to be a promising way to suppress the carbide formation and enable the use of Mg-Al matrix with appropriate Al content.