Silicon carbide fiber reinforced siliconoxycarbide (SiC_f/Si-O-C) ceramic composites were fabricated via precursor infiltration and pyrolysis using polysiloxane. The effects of the inert fillers (IF) SiC, SiO₂and SiO₂cenosphere on flexural strength, density, coefficient of heat conductivity of SiC_f/Si-O-C composites were studied. Microstructure analysis shows that the properties are mainly influenced by the inner fillers due to the interface structure and the relative density.