C/C-SiC composites were rapidly fabricated using C/C with four different porosities in the range of 12.4%~45.7% and silicon by reactive-melt-infiltrated (RMI) method. The influence of the C/C porosity on the Si infiltration during the processing and on the microstructure and mechanical properties of the resulting C/C-SiC were investigated. The results show that β-SiC was formed by Si/C reaction and free Si remained in the composites. A higher porosity of C/C leads more Si infiltrating to the preform and produces higher density of C/C-SiC with lower porosity. The flexural strength of the composites was strong influenced by the matrix content and the interface between different phases. C/C-SiC derived from C/C with 24.8% porosity has the highest flexural strength (325.1MPa).