The thermal conductivity of monolithic Y-Sm/alpha-SiAlON was evaluated using experimental data and finite element analysis. The thermal diffusivities of Y, Y-Dy, and Y-Ce/alpha-SiAlON ceramics were also investigated experimentally for comparison. The maximum achievable thermal conductivity of Y-Sm/alpha-SiAlON has been calculated by the linear extrapolation of the temperature-based experimental inverse diffusivity data and was used for the numerical calculations. Two-dimensional model microstructures were built on the base of real microstructure images and applied for calculations. Experimental data and numerical calculations were compared for Y-Sm/alpha-SiAlON, and it was revealed that both results are in good agreement.