A thermodynamic theory of sintering of granular systems for modeling of such processes as compaction, coalescence of pores and clustering of granules is proposed. For describing a thermodynamic state, generalized coordinates, a state function, and a constitutive equation are determined. It is shown, that the second law of thermodynamics imposes restrictions on the state function and Helmgoltz’s free energy. For two models, Helmholtz’s free energy is determined and the heat equations are formulated. Further development of the theory is specified.