Abstract: Carbon/carbon composites with densities of 1.67-1.72 g/cm³ were prepared by film boiling chemical vapor infiltration at 1 000-1 100℃ using xylene and LaCl₃ as carbon source and catalyst, respectively. The influence of the catalyst content on the density, pyrocarbon (PyC) microstructure and mechanical properties of the composites was studied by polarized light microscopy, scanning and transmission electron microscopy and mechanical testing. Results show that the PyC deposition rate is remarkably increased by the presence of the catalyst. The microstructure of the PyC changes from a rough laminar (RL) to isotropic (ISO) with increasing catalyst content from 0 to 15 wt%. The density exhibits a maximum with the catalyst content at 3 wt% while the flexural strength and interlaminar shear strength exhibit maxima of 230.7 and 36.6 MPa, respectively, with the catalyst content at 6 wt%. Nanofilamentous carbon is found in the PyC when the catalyst content is above 3wt%. The catalyst particles are encapsulated by the PyC, resulting in the formation of a hybrid matrix consisting of RL and ISO PyCs. The thickness of the ISO layer increases with the catalyst content. The flexural and interlaminar shear strengths of the composites are increased by 8.1-33.0% and 15.3-55.7%, respectively, compared with the values for samples without the catalyst. The toughness of the composites is increased, but the flexural and interlaminar shear strengths are decreased by high temperature treatment. The maximum degradation of the flexural and interlaminar shear strengths is found at acatalyst content of 15 wt%, and these are 18.6 and 14.4% lower than the values for samples with no catalyst.