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The stability of closed pores in two and three dimensions has been discussed and it is found that the stability of pores in two dimension can be determined mathematically from their particle coordination number and dihedral angle; while those in three dimension can be approximately determined by a spherical pore model. This model is set up by first excluding the effect of interface tension, so the pore was supposed to be spherical, and then the tensile stress arising from the interface tension was allowed to act on this hypothesized spherical pore. On the basis of the spherical pore model, pore microstructure models for real powder compacts were set up and the densification equations for the intermediate and final stages of sintering were derived. The criterion for pore shrinkage, and the effect of pore size distribution and green density were discussed according to the derived equations. The densification equations for pressureless solid state sintering can be easily extended to describe the densification behaviour during hot-pressing or hot-isostatic-pressing. Densification characteristics in liquid state sintering were also considered from the result of solid state sintering.