Mesoporous carbon-silica (C-SiO2) nanocomposites with different C/SiO2 molar ratios were used as precursor for fabricating silicon-based ceramics. Different silicon carbide nanostructures were synthesized by carbothermal reduction of mesoporous C-SiO2 nanocomposites via different heat treatments under argon. X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and nitrogen adsorption-desorption analysis were used to characterize C-SiO2 nanocomposites and SiC products. The major morphologies formed from the mesoporous C-SiO2 nanocomposites were nanoparticles and nanofibers. With higher quantity of P123, which is the surfactant for the mesopores, the BET surface area and pore volume increased, inducing a decrease of nanofibers. The mesoporous precursors were also heated at 1200 °C for 15 hours to make a nearly dense structure and then heated to the final temperature. The products were almost nanoparticles which had a larger size than those heated directly to 1450 °C. Therefore carbothermal reduction of mesoporous C-SiO2 precursors appears to be an effective means of accelerating the reaction and controlling SiC nanostructures.