The fabrication process of $Al_2O_{3p}/Al$ composite by in-situ process was investigated. The effects of processing variables such as addition type and content of Mg, processing temperature and time on the infiltration behavior of molten Al, microstructure and hardness were investigated. When the pure Al was infiltrated into mixtures of Mg and $Al_2O_3l$ powder, processing temperature required to spontaneous infiltration was decreased, and the content of Mg was the most powerful variable for infiltration of molten Al. But when the Al-Mg alloy was infiltrated into $Al_2O_3l$ particles, infiltration ratio indicated nearly same value regardless of Mg content in alloy and processing temperature, and critical processing temperature required to spontaneous infiltration was $800^{circ}C$. The $Al_2O_{3p}/Al$ composites which were fabricated by mixtures of Mg and $Al_2O_3l$ powders resulted in high hardness value, but hardness values were scattered due to non uniform dispersion of $Al_2O_3l$ particles by excessive reaction of Mg.