This paper deals with modeling of grain growth and pore shrinkage during sintering. The simulation takes as the theoretical basis the curvature of the grain boundaries and the pore surfaces and their mobilities. The movement of the grain boundaries is calculated for each grain separately using the flux equation for grain growth j = c(k)(1/r(2)-1/r(1)), c(k) being a rate constant, r(1) and r(2) effective radii for each grain boundary. The modification of these boundaries in a micro structure is calculated in two and three-dimensional space starting with a Voronoy mosaic at the beginning of sintering. Two kinds of pores are distinguished: sphere like pores situated between two grains and non spherical pores between at least three grains. The results of several computer runs are shown and discussed.