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The effect of CeO₂ content on the microstructure and properties of SiCp/Al-Si composites prepared by powder metallurgy was studied, and the mechanism of CeO₂ in composites was deeply analyzed. The results show that the addition of the appropriate amount of CeO₂ can refine the Si particles and improve the tensile properties of the SiCp/Al-Si composites. As the CeO₂ content increases from 0 to 0.4 vol%, the particle size of the Si phase shows a tendency to decrease first and then increase, while the tensile strength, yield strength, and elongation of the composites show a trend of first increasing and then decreasing. When the CeO₂ content is 0.2 vol%, the refining effect of CeO₂ and the tensile properties of composites are the best. The fracture mode of SiCp/Al-Si composites with a rare earth addition is a mixed fracture. There are three main mechanisms for CeO₂ in SiCp/Al-Si composites. One is when CeO₂ serves as the nucleation substrate of Si phase to refine Si particles. The second is when CeO₂ reacts with the alloying elements in the aluminum matrix to form a new phase, CeCu₂Si₂, which can not only play a role of dispersion strengthening, but also improve the bonding strength between Al matrix and Si particles. The third is the pinning effect of CeO₂ and CeCu₂Si₂ particles on grain boundaries or phase boundaries to refine aluminum grains.