During the last stage of sintering of ZnO, a decrease in bulk density is accompanied by an increase in pore size and a decrease in the number of pores. Examination of the microstructures indicated that the pores are almost always situated at the grain boundaries and at the corners. The dominant pore size increases with time raised to the 1/3 power. The apparent diffusion coefficients calculated from the pore growth rate are in good agreement with lattice diffusivity for zinc. It is postulated that gas diffusion takes place along the boundaries during pore growth.