Abstract: SiC/Al composites were fabricated by pressureless infiltration technique.The effect of pore-making agent on porosity of porous preforms was studied.The results show that infiltration of liquid Al into porous preforms relies on various obstructions and force of infiltration,especially on the properties of the porous preforms.Composities with low porosity and high properties can be obtained by adjusting porosity and pore structures.Practice shows that the higher the porosity of porous preform and the larger diameter of pore in porous preforms,the lower the porosity of composites.