The interfacial reaction of liquid stirred cast SiC
P and SiO
2(P) reinforced Al4wt%Mg matrix composites was studied by means of analytical transmission electronic microscope. The results show that the fine MgO particles are formed on the surface of as received SiC particle and oxidized one. The quartz particles reacted with magnesium in matrix to form MgAl
2O
4. And the reaction mechanisms are explained respectively with the diffusion theory.