The sintering stress is related to the thermal stability of porous structure. The sintering stress for a given porous structure in equilibrium can be calculated by three methods theoretically; the energy difference method, the curvature method, and the force balance method. The sintering stresses by three different methods were exactly the same for the idealized porous materials in equilibrium, in which the pore surface had a constant curvature at any point. The porous material does not spontaneously shrink when the sintering stress becomes zero or negative. The sintering stress will be used to design optimal porous structures with improved thermal stability.