Abstract: The carbon nanotubes (CNTs) were in-situ grown on carbon fiber felts and the hybrid felts were densified by chemical vapor infiltration (CVI) to prepare CNTs-C/C composites.The effects of catalyst content on the growth of CNTs and the effects of CNT content on the flexural properties of CNTs-C/C composites were investigated. The results show that catalyst content has a great influence on the yield of CNTs. More CNTs were obtained with the catalyst content increasing. The catalysts used for CNT growing deteriorate the flexural properties of C/C composites. The CNTs changes the infiltration behaviors of pyrocarbons and induces the formation of abundant small-sized spherical or cone-shaped pyrocarbons, which inhibits the formation of annular cracks in pyrocarbon matrix. The CNTs with a suitable content can increase the flexural strength and modulus and improve the fracture behavior of C/C composites.