Abstract: Preforms made of carbon fibers and sacrificial tungsten fibers as a pore forming agent were repeatedly infiltrated with polycarbosilane and pyrolysed for 14 cycles, followed by aqua regia leaching to remove the tungsten fibers to form porous C/SiC composites. The pore structure, mechanical properties and permeability of the porous C/SiC composites to water and kerosene were respectively studied by a scanning electron microscope, a universal materials testing machine and home-made permeation equipment based on a fluid flow model in capillaries. Results showed that the pores of the porous C/SiC composites were mostly straight and that the open porosity can be controlled by the volume fraction of tungsten fibers used. The bend strength, Young’ s modulus and fracture toughness were 358MPa, 124GPa and 16.7MPa · m^1/2 respectively when the open porosity of the composite was 23.5%. The fluids permeated through the porous C/SiC composites by linear laminar flow（the osmosis rate value is 1.02×10^-3mm²）. The composites exhibited tough fracture behavior and the presence of pores didn’t lead to a significant deterioration of their mechanical properties.