Sintering behaviors of ceria powders with large and fine particle size and gadolinia-doped ceria powders with a fine particle size have been studied by measuring density and grain size as a function of sintering time. Densification rate and grain growth rate of ceria with a fine particle size were larger than those with a large particle size. Grain growth rate of gadolinia-doped ceria was much smaller than that of ceria, while densification rate was considerably larger. Inhibition of grain growth observed in gadolinia-doped ceria was explained by the solute drag model due to a space charge effect. The fact that the densification rate in gadolinia-doped ceria was considerably larger than that in ceria suggests that the rate determining step of densification is the diffusion of oxygen. In gadolinia-doped ceria, the diffusion of oxygen is larger, the shrinkage of internal pores is considered to proceed faster due to larger mobility of oxygen and then the densification rate becomes larger than in ceria.