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Preforms containing carbon nanotubes (CNTs) were fabricated by in situ grown in unidirectional carbon cloths, and densified by chemical vapor infiltration (CVI) to prepare CNT reinforced C/C composites (CNT-C/C composites). The density change of obtained composites during CV! was investigated. The results show that densification rate of CNT-C/C composites is higher than that of pure C/C composite. Densification rate has the maximum value when the CNT content is 5 wt%. Polarized-light optical microscopy, SEM and TEM were used to analyze the effect of CNTs on the structure of obtained composites. The results show that the existence of CNTs can not only contribute to the formation of high-textured pyrolytic carbon, but also increase the number of interfaces and form the complicated interface in C/C composites. This structure change leads to higher flexural properties of CNT-C/C composites than that of pure C/C composite. (C) 2010 Elsevier B.V. All rights reserved.