Abstract: The carbon/ carbon (C/ C) composites within the mat rix-crack microst ructure were prepared by pyrolyzing carbon fiber/ phenolic resin composite which was compacted by hot pressing. The material density of C/ C composites was about 1. 27 g/ cm³ , and the open porosity was 27 % af ter pyrolysis at 900 ℃. The alterations of density ,open porosity and mass loss during pyrolysis from 200 ℃to 900 ℃were investigated. The microst ructural evolutionof phenolic resin2based C/ C composites during pyrolysis was analyzed. The phenolic resin mainly decomposed f rom400 ℃to 700 ℃, and the decomposition rate was 4 times that of other temperature ranges. A high amount of regularly spaced segmentation cracks formed above 400 ℃, and further increased and expanded with elevated temperature. A microscopic network of the characteristic interconnecting crack pattern within C/ C composites was developed af ter pyrolysis at 900 ℃. The total pore volume of C/ C composites was 0. 17cm³ / g , and 81 % of the poreswere in the radius range of 122. 190～2. 440μm.