Metal matrix composites (MMCs) that consisted of Al-Mg matrix reinforced with initially added BN particles were fabricated using the pressureless infiltration technique, and their oxidation behavior was investigated at 500 and 550oC in air. Initially added BN particles were partly consumed to make AlN, as another reinforcing particles. Another reaction product was MgAlB2 dispersoids. Despite of the presence of these reinforcing particles, the oxidation resistance was not good owing to the formation of MgO-rich scale due to the preferential oxidation of Mg in the MMCs. Detailed microstructures and oxidation mechanism of the prepared MMCs are proposed.