The high thermal conductivity of SiC single crystals makes them crucial in fields such as renewable energy and aerospace. Nevertheless, owing to the phonon scattering resulting from crystal boundary, the reported thermal conductivity of SiC ceramic is always well below the intrinsic value. For the SiC ceramic derived from β-SiC (3 C-SiC) starting powder, the β→α phase transformation (i.e. 3 C→4 H) happened imposes significant influences on its microstructure. Therefore, systematic studies of contributions from hetero-/homo- phase boundaries are necessary for understanding the thermal conductivities of resulting SiC ceramic, providing further guidance to the phase transformations control. In this work, the thermal conductivities of both β/β and α/α homophase boundary, as well as β/α heterophase boundary, are systematically investigated. Further, it is found that the highest thermal conductivity is reached at sintering temperature of around 2000 °C, where about 90% of SiC has been transformed from β to α content.