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A new fabrication method of particulate reinforced magnesium matrix composites using a spontaneous infiltration phenomenon was developed. By mixing infiltration agent powder with reinforcement powder, the magnesium melt spontaneously infiltrated into the powder mixture. In this study, pure magnesium, SiO2 and SiC particle (SiCp, 1.2,2,3,4 and 8 μm) powders were used as the matrix metal, infiltration agent and reinforcements, respectively. Experiments to clarify the conditions in which the spontaneous infiltration occurred were carried out under pure argon atmosphere at 973 K. Furthermore, the infiltration velocity was measured. Spontaneous infiltration occurred in the 2,3,4 and 8 μm SiCp systems. A minimum SiO2 content necessary for spontaneous infiltration existed and it depended on SiCp diameter. Microstructural observation of the composites revealed that SiCp dispersed homogeneously, even though the diameter of SiCp was fine, and MgO and Mg2Si reaction products were observed. During infiltration, a temperature rise was observed, caused by Mg–SiO2 thermit reaction. Consequently, spontaneous infiltration was caused by improvement of wettability between the magnesium melt and SiCp, which resulted from Mg–SiO2 thermit reaction at the infiltration front.