SiCp/Al composites based on different matrix elements Mg and Si content were fabricated by pressureless infiltration. Through microscope investigation by SEM and TEM, density examination, coefficient of thermal expansion (CTE) and thermal conductivity measuring by Netzsch DIL 402EP and Netzsch LFA 447 Nanoflash, the effects of Mg and Si in the aluminum on the thermo-physical properties of SiCp/Al composites was studied and analysed. The results showed that , adding Mg element in matrix enhaced the densification and thermal conductivity of composites, but meanwhile increased the CTE of SiCp/Al composites. When Mg element content was in higher than 6.3wt%, CTE of SiCp/Al composites increased obviously. Adding Si element in matrix markedly reduced the CTE of composites , but meanwhile decreased thermal conductivity of composites. When Si element content was higher than 5-9wt%,thermal conductivity of composites declined obviously. Adding Mg element in matrix improved the interface wet ability between molten aluminum and SiC particles, and adding Si element in matrix could control the formation of A14C3 which is a detrimental interface reaction product during the pressureless infiltration processing. Therefore, adding appropriate Mg in company with Si in matrix will be more benefit to enhance the integer properties of SiCp/Al composites.