This study investigates the thermal and electrical properties of SiC ceramics with a combination of Y2O3 and rare-earth-oxide additions as sintering additives, by comparing four types of SiC starting powders varying in particle size and chemical composition. The powder mixtures were plasma-activation sintered to full densities and then annealed at high temperatures for grain growth. The thermal conductivity and electrical resistivity of the SiC ceramics were measured at room temperature by a laser-flash technique and a current-voltage method, respectively. The results indicate that the thermal conductivity and electrical resistivity of the SiC ceramics are dependent on the chemical composition and particle size of the starting powders. The thermal conductivities observed for all of the annealed materials with a rare-earth La2O3 sintering additive were > 160 W-.(m(.)K)(-1), although low electrical resistivity was observed for all materials, in the range 3.4-450 Ohm (.)cm. High thermal conductivity, up to 242 W-.(m(.)K)(-1), was achieved in an annealed material using a commercial 270 nm SiC starting powder.