Two C/SiC composits reinforced respectively by T300 and JC2# carbon fibers were fabricated by means of polymer infiltration and pyrolysis (PIP) with polycarbosilane (PCS) as precursor. The composite reinforced by JC2# fiber had perfect mechanical properties, displaying flexural strength of 662MPa and fracture toughness of 19.5MPa. m^1/2, whereas T300 C/SiC composite had low strength and high brittleness, with its flexural strength and fracture toughness were less than one fourth of those of JC2# C/SiC composite. It was found that the fundamental factor to the poor mechanical properties of T300 C/SiC composite was the severe degradation of the fiber during PCS pyrolysis; the other important one was attributed to the strong interfacial bonding between the fiber and agent in the composite.