Micro-/mesoporous SiOC bulk ceramics with high surface area and bimodal pore size distribution were prepared by pyrolysis of polysiloxane in argon atmosphere at 1100 degrees C-1400 degrees C followed by etching in hydrofluoric acid solution. Their thermal behaviors, phase compositions, and microstructures at different nano-SiO2 filler contents and pyrolysis temperatures were investigated by XRD, SEM, DSC, and BET. The SiO2 fillers and SiO2-rich clusters in the SiOC matrix act as pore-forming sites and can be etched away by HF. At the same time, the SiO2 filler promotes SiOC phase separation during the pyrolysis. The filler content and pyrolysis temperature have important effects on phase compositions and microstructures of porous SiOC ceramics. The resulting porous SiOC bulk ceramic has a maximum specific surface area of 822.7m(2)/g and an average pore size of 2.61nm, and consists of free carbon, silicon carbide, and silicon oxycarbide phases.