Abstract: Interfacial reactions mechanism between SiO₂ grain and Al-Mg matrix is investigated, and20wt% SiO₂/Al-Mg composites were fabricated by powder metallurgy. As a result, MgAl₂O₄, MgO, Mg₂Si and a little Al and Si were created in formerly SiO₂ grain. MgAl₂O₄ is in an all irregular shape, and MgAl₂O 4 is closed about Al. The structure of MgO, Mg₂Si and Si is slice layer. There is no SiO₂ in the formerly SiO₂ grain, which shows that whole SiO₂ had been reacted with Al-Mg matrix by 620℃sinter for 30min. Si created by reaction of SiO₂ and Al-Mg matrix was pushed into Al-Mg matrix. There are Al, Si and Mg₂Si in the matrix. Mg₂Si is in a grain shape, its size being smaller than 0.2μm. There is no simple Mg in the matrix because the Mg there had been reacted to form Mg₂Si.