A change of thermal conductivity caused by brittle damage evolution in concrete subjected to mechanical load is analysed in the paper. Considerations are illustrated by own experiment in which thermal conductivity of high performance concrete is compered for the loaded and not loaded laboratory-samples. A quantitative assessment of the experiment is made on the basis of reciprocity theorem formulated for the heat transfer problem. To simplify the considerations it is assumed that damage in material is described by the scalar dimensionless parameter ω.