Cerium oxide is a very useful base material used as catalyst supports, ion conductors and gas sensors. It is also well known that ceria-based materials are difficult to densify below 1500degreesC. However, a small amount of Fe doping obviously promotes the densification rate and reduces the sintering temperatures. In this study, the early stage sintering mechanisms of undoped and Fe-doped CeO2 were investigated, based on two sintering models. We confirm that pure CeO2 exhibits volume-diffusion controlled sintering, while a viscous flow mechanism dominates the early stage sintering of Fe-doped CeO2.