Cerium oxide (CeO2) is a promising material that has potential for use in a number of applications, such as resistive-type oxygen sensors and solid oxide fuel cells. In this work, the sintering behavior of hydrothermal synthesized nano-size CeO2 powders and chemical precipitated and commercial micron-size CeO2 powders were investigated by continuous monitoring of the shrinkage kinetics. The results demonstrated that during the high temperature sintering process a partial redox reaction of ceria occurred, i.e., a fraction of Ce4+ was reduced to Ce3+, and oxygen gas was released. The redox reaction influenced the sintering behavior of CeO2, resulting in a decrease in density and microcracking for the hydrothermal synthesized nano-size CeO2 powder compacts and sagged points in the sintering curves for the chemical precipitated and commercial micron-size CeO2 powder compacts. It was found by scanning electron microscopy that the partial redox reaction of ceria produced additional pores in the powder compacts during the sintering process and thus much higher temperatures were needed to achieve high density.