BN/Al(-Mg) metal matrix composite (MMC) was fabricated by the pressureless infiltration technique. The phase characterizations of the composites were analyzed using the SEM, TEM, EDS and EPMA on reaction products after the electrochemical dissolution of the matrix. It is confirmed that aluminum nitride (AlN) was formed by the reaction of Mg 3N 2 and Al alloy melt. Plate type AlN and polyhedral type Mg(-Al) boride were formed by the reaction between Mg 2N 2, BN and molten Al in the composite. The reaction mechanism in the fabrication of BN/Al(-Mg)MMC was derived from the phase analysis results and the thermodynamic investigation.