Si3N4/Al-Mg composites reinforced by ceramic interpenetrating network structure had been fabricated via pressureless infiltration technology. The matrix and the reinforcement phase, form an interconnected interpenetrating network structure. The Al-Mg/Si3N4 system exhibits an excellent wettability under moderate conditions. The increasing of Mg content (2-10 wt%) resulted in an increased amount of infiltration, once Mg content beyond 10 wt% has an adverse effect. Light chemical reaction occurs in the interface of Al-Mg/Si3N4 system and the reaction productions reduce the surface tension of melt and impulse the advance of infiltration. Infiltration temperature and infiltration time were the key parameters, which turn into the infiltration impetus. The appropriate infiltration temperature is 1050 degrees C and the corresponding infiltration time is 15 min, prolonging the infiltration time continuously has no significance.