Carbon/carbon composites (C/C composites) possess superior characteristics of low density, high strength, extremely low coefficient of thermal expansion, and high fatigue resistance. In carbonization process, the high-temperature pyrolysis made of carbon, hydrogen, oxygen, and other elements results in a lot of voids and cavities generated in the interior of C/C composites. Therefore, the C/C composites are densified to fill the voids by using repeated impregnation. But densification is a time-wasting and complex process, which increases production costs in the manufacturing process. In this study, the multiwall carbon nanotubes (MWNTs) were adopted as a reinforcement material for C/C composites to reduce the existence of voids or cavities and enhance the mechanical properties of C/C composites. According to the experimental results, the CNT-added C/C composite containing 1.2wt% CNT possesses the greatest flexure strength, flexure modulus, and interlaminar shearing strength. Plus, the above-mentioned strength and modulus are increased by 23%, 19.2%, and 30%, respectively. © 2012 Yi-Luen Li et al.