SiCp/Al composites of 55%~70% (volume fraction) were prepared by pressureless infiltration to investigate the reaction mechanism, morphology and the thermo-physical properties of the prepared composites. X-ray diffraction (XRD) and thermodynamic analyses show that the main interface reaction may be SiO2(s)+Al(l)+MgO(s)→MgAl2O4(s)+Si(s) in the preparation process of the composites. The enhancing of Si activity can obviously suppresses the formation of harmful interface reaction product Al4C3. Metallographic observation shows that the microstructures of the composites are homogenous and dense, infiltration defects are liable to occur in the preparation process. The thermo-physical tests show that the best thermal conductivity (TC) and the coefficient of thermal expansion (CTE) for the prepared SiCp/Al composites with less infiltration defects and dense structure are 170.2 W/(m·K) and 6.64×10-6 K-1, respectively.