Three kinds of C/C composites with different pore structures were infiltrated by silicon. SEM， XRD，micro-CT and mercury intrusion porosimetry methods were adopted to characterize the microstructure， phases and pore-structure changes before and after the infiltration.The apparent porosity of all infiltrated samples was below 2%，indicating high densification degree of them.During the infiltration process，the carbon fibers in the C/C composites can be protected by resin carbon which formed by the pyrolysis of high-char-yied resin or PyC fabricated by chemical vapor depostion method，and the bending strength increases by 1.5 and 2.5 times， respectively.While the carbon fibers in the composites are not protected by the matrix， the bending strength of the samples decreases by 41%.