3-D braided C fiber preform was used to reinforce SiC matrix by polymer infiltration and pyrolysis (PIP). The effect of PCS pyrolysis process on the uncoated carbon fiber was studied. During the pyrolysis, amorphous SiCxOy and some free silicon yielded. The Si element diffused into the C fiber from the matrix because of the concentration gradient at high temperature and destroyed the intrinsic structure of the uncoated C fiber. At the same time, the free Si reacted with the uncoated C fiber. Thus, strong bonding between the fiber and matrix was formed. As a result, bending strength of the composite was decreased.