Some quantification methods were developed for microstructure characterization of pyrocarbon (PyC) in carbon/carbon (C/C) composites fabricated by chemical vapor infiltration. Firstly, the feature parameters were strictly defined based on 002 lattice fringe images in high-resolution transmission electron microscopy. They were illustrated with the diagram. Image analysis was adopted to measure or calculate the feature parameters of describing individual fringe or expressing the relationship between fringes. An improved comprehensive parameter, crystallinity factor, was introduced to indicate the crystallization of PyC. Then, judgment rules of microcrystal units were considered from three aspects: interlayer spacing, stacked position, and parallelism between layers. The latter two were analyzed by drawing geometrical figures. Finally, the extracted parameters were displayed in statistical histograms, and also compared with the measurements from manual observation, X-ray diffraction, and selected area electron diffraction. The results were in general agreement. The developed algorithms were performed automatically by the computer. They are especially applicable to analyze the microstructure evolution of C/C composites in graphitization process. The extracted feature parameters supply the basis for studying mechanism of graphitization and interfacial structure of PyC.