C/SiC, C/Si-O-C and C/C composites reinforced with T300 carbon fiber were fabricated via polycarbosilane (PCS), polysiloxane (PSO), and phenolic resin precursor polymers infiltration/pyrolysis, respectively. Flexural strength and fracture toughness of the composites were evaluated. The results showed that all the composites had poor mechanical properties, less than 160 MPa in flexural strength and 5 MPa•m1/2 in fracture toughness. Deep investigation illuminated that the fiber was damaged severely during the preparation of the composites, especially in the first cycle of precursor pyrolysis. Great degradation of the fiber has relationship with coarsening of the microstructure. Bad in-situ strength of the fiber resulted in poor performance of the composites.