Mesoporous silica-carbon nanocomposites (C-SiO(2)) were synthesized for the fabrication of highly crystalline silicon carbide (SiC) nanoparticles and nanofibers via carbothermal reduction. SiC nuclei were introduced into the mesopores as seeds by infiltration of preceramic precursor polycarbosilane (PCS) prior to the heat treatment of carbothermal reduction. When PCS with a mass percentage of 11-13% was infiltrated into the mesoporous C-SiO(2), SiC nanofibers and nanoparticles were produced at 1450 °C, even in the sample with low carbon content. The major morphology formed from the mesoporous C-SiO(2) nanocomposites without PCS infiltration was nanoparticles, while nanofibers dominated in the products of PCS-infiltrated compositions. The results indicate that the conversion of PCS into SiC nuclei in mesopores prior to carbothermal reduction has facilitated the formation of SiC nanofibers. Therefore infiltration of seeds into mesopores of C-SiO(2) precursors appears to be an effective means of accelerating the reaction and controlling SiC nanostructures.