The fabrication of magnesium matrix composites using spontaneous infiltration phenomenon becomes possible by adding SiO2 powder (infiltration agent) to SiCp powder (reinforcement), even at relatively low holding temperature and under argon atmosphere. It is probably because the wettability between Mg melt at the infiltration front and reinforcement is improved by Mg/SiO2 thermit reaction heating. In this paper, the infiltration velocity is measured. Especially the effects of SiCp diameter from

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... to 8μm and SiO2 content are discussed. The results show that the infiltration velocity is in the range from 0.01 to 0.7mm・s-1, and it depends both on SiCp diameter and on SiO2 content. The effect of SiO2 content on infiltration velocity is complex, i. e. the velocity increases with increasing SiO2 content up to 15mass%, and then decreases steeply. This decrease is probably caused by the increased infiltration resistance with MgO and Mg2Si, products of Mg/SiO2 reaction. Up to 15mass%SiO2, the increasing of infiltration front temperature by increasing SiO2 content is efficient, resulting in faster infiltration velocity. Above this content, the effect of infiltration resistance by the reaction products becomes more significant than that of the temperature increase, resulting in slower velocity.