[en] The effects of microstructure of boron carbide on physical properties were studied to determine limitations on the uses of the material under irradiation. A thermal conductivity model was developed including the total porosity and the grain size of the boron carbide pellets. The effect of stoichiometry and ₁₀B enrichment were also investigated. The heat transport from room temperature to 1800₀C is mainly the results of diffusion of phonon and collisions phonon-phonon. Thermal conductivity after fast neutron irradiation to 80.10₂₀ capture.cm^-₃ became lower and the mechanism of heat transport was affected. After irradiation thermal expansion and crystal cell volume of boron carbide were higher. Then, the melting point depends on stoichiometric ratio because the boron carbide phase is a solid-solution. On the carbon-rich side with more than 20 at. % of C, the variation of the melting point with composition could reached 200₀C