A new simulation method for sintering in three-dimensional powder packings is presented. The model is based on two-particle sintering interactions and is able to describe mass transport mechanisms based on grain-boundary diffusion as well as on lattice diffusion. It is shown that the simulation method provides an adequate description of structural reorganization effects which occur during the early stage of sintering. Simulation results are presented for the sintering of a random packing and of some crystalline-type packings in which various initial defects have been created.