The pressureless infiltration process to synthesize a silicon nitride composite was investigated. An Al-2wt%Mg alloy was infiltrated into silicon nitride preforms in the atmosphere of nitrogen. It is possible to infiltrate the Al-2wt%Mg alloy in silicon nitride preforms. The growth of the composite with useful thickness wasfacilitated by the presence of magnesium powder at the interface and by flowing nitrogen. The microstructure of the Si₃N₄-Al composite has been characterized using scanning electron microscope.During the infiltration of Si₃N₄ preforms, Si₃N₄ reacted with aluminium to form silicon and AlN. Thesilicon produced during the growth consumed in the formation of MgSiAlO, MgSiAlN and Al_3.27Si_0.47 type phases. The growth of the composite was found to proceed in two ways, depending on the oxide content in the initial preforms. First, less oxide content preforms gave rise to MgAlSiO and MgAlSiN type phases after infiltration. Second, more oxide content preforms gave rise to AlN-Al₂O₃ solid solution phase(AlON). The AlON phase was only present in the composite, containing 10% aluminium in the silicon nitride preforms before infiltration.