In this work, the effects of carbon nanotubes (CNTs) on the microstructure evolution, thermal conductivity, and mechanical properties of C_f/SiC composites during chemical vapor infiltration (CVI) densification were investigated in detail. Compared with composites without CNTs, the thermal conductivity, flexural strength, flexural modulus, fracture toughness, interfacial shear strength, and proportional limit stress of specimens with CNTs of 4.94 wt% were improved by 117%, 21.8%, 67.4%, 10.3%, 36.4%, and 71.1%, respectively. This improvement was attributed to the role of CNTs in the division of inter-layer pores, which provided abundant vapor growth sites for the ceramic matrix and promoted densification of the whole composite. In addition, the high thermal conductivity network formed by the overlap of CNTs and the rivet strengthening effect of CNTs were beneficial for synergistic improvement of thermal conductivity and mechanical properties of the composites. Therefore, this study has practical significance for the development of thermal protection composite components with enhanced thermal conductivity and mechanical characteristics.