We present an in-depth study of the effect of carbon nanotube (CNT) extending length on microstructural and mechanical behavior of carbon/carbon (C/C) composite. High-purity CNTs with controlled extending length were in situ grown on the surface of carbon cloths using injection chemical vapor deposition (ICVD) by varying the reaction time. Microstructure analysis shows that compared with the CNTs of short extending length which only change the pyrocarbon (PyC) close to fiber surface, CNTs with long extending length can strongly affect the deposition behavior of PyC during chemical vapor infiltration and modify the whole matrix PyC. Mechanical tests reveal that CNTs with long extending length are more beneficial to enhance the interlaminar shear strength and in-plane compressive strength of the composites, while the reactive conditions during ICVD degrade the carbon fibers and lead to the decrease of flexural strength. Our work demonstrates that it is necessary to make CNTs long enough as well as to prevent strength degradation of fibers, if we want to largely increase delamination resistance and through-thickness properties without compromising in-plane performance of C/C composites. (C) 2015 Elsevier B.V. All rights reserved.