Short fiber reinforced AlSi12CuMgNi composites have real applications as engine components in automobile industry. The addition of short fibers can raise the interfacial reactions between the fiber and matrix during casting. The reaction products can change the composition and further alter the physical metallurgy of matrix, such as microstructure, aging behavior and mechanical properties. The presented work investigates the interfacial reactions in the Saffil®- and Kaowool®-fiber reinforced AlSi12CuMgNi composites. The interfacial reactions are analyzed using SEM and TEM techniques. The effects of fiber composition on the interfacial reaction will be discussed. It is shown that the interfacial reaction largely depends on the fiber composition, especially the content of SiO₂ in the fiber. The interfacial reaction occurring during casting can drastically influence the subsequent mechanical behavior of the investigated composites. No ageing phenomena are noted in the Kaowool®-fiber reinforced AlSi12CuMgNi composite. However, a typical ageing peak can be observed in the Saffil®-fiber reinforced composite. The relationship between the interfacial reaction and mechanical behavior will be discussed based on the experimental results. The ageing hardening deteriorated due to the depletion of alloying element magnesium caused by the interfacial reaction.