Taking polycarbosilane and antimony-modified polysilane for precursors and drawing lessons from the transformation process of high temperature graphitization of spare free carbon in SiC materials, the polymer-derived SiC nanocomposites were prepared by employing hot-press sintering, precursor infiltrating-cracking methods and annealing. The phases and microstructures were characterized and analyzed by using SEM, TEM, XRD and Raman test methods. The thermoelectric parameters of samples, such as thermal conductivity, electrical conductivity and Seebeck coefficient, were studied. Results show that the dense SiC nanocomposites acquired are n-type thermoelectric materials. The thermal conductivity is suppressed at the range of 4～8W/(m?K) due to the function of nano-graphite. At the annealing temperature of 1600°C, the thermal conductivity reduces while both the electrical conductivity and the Seebeck coefficient increase, and ZT value of SiC nanocomposites derived from precursor reaches to 0.0028（650°C）, which is higher than that of SiC/C composites or nanocomposites ever reported.