The effects of processing parameters such as addition type and Mg content on the fabrication of Al2O3P/Al composites by in-situ reaction process of molten Al were investigated in terms of infiltration behavior of molten Al and the variation of the microstructure and hardness of the composites. It was ascertained that additional Mg content is the most important parameter for the infiltration of molten Al. Furthermore, increase in additional Mg content and processing temperature resulted in a high infiltration ratio of molten Al and a high hardness value of the composites. However, these caused a greater scatter of the hardness value of the composites due to the nonuniform dispersion of the reinforcement phase, which arose from an excessive reaction of Mg.