Two carbonized oxidized polyacrylonitrile fiber (OPF) felts and one polyacrylonitrile-based carbon fiber (CF) felt were used as preforms to prepare two kinds of carbon/ carbon composites by chemical vapor infiltration,and the effect of fiber type on the microstructure and mechanical properties of the composites were investigated. The microstructure was characterized bypolarized light microscopy and Raman spectroscopy and the mechanical properties were characterized by nanoindentation and three-point bend tests. The two carbonized OPFs are surrounded by a darklaminar layer about 1. 4-2. 6 μm thick followed by a rough laminar layer of about 10. 2-11. 6 μm, while the CFs are surrounded by a smooth laminar layer about 8. 8 μm thick and arough laminar layer of about 4. 4μm. Nanoindentation indicates that the modulus and hardness of the carbonized OPFsare obviously lower than those of the CFs, and the modulus and hardness of the matrix decrease with increasing extinction angle. The low modulus of the matrix and the OPFsresult in a decrease of the tensile and flexural strength by about 14. 5%-24. 2% and 7. 3%-15. 4% and a decrease of the tensile and flexural modulus by about 9. 7%-19. 8% and 15. 1%-18. 6%, respectively, for the OPF-derived composites compared with the CF-derived composites. However, for the OPF-derived composites the ductility factor increases by about 224%-235% because of the high content of rough laminarcarbon and the obvious shrinkage of the OPFs after graphitization. Meanwhile, a modelin-volving the three components in the composites is proposed to predict their tensile modulus, which shows deviations between experimental and predicted results below 9. 9%.