Rare earth (RE) metals have often been used as dopants to improve the catalytic activity of ceria. However, their exact role in the activity of ceria catalyst has not been clearly identified. Combining experimental and theoretical approaches, we extensively investigate CO oxidation as a model reaction on RE-doped ceria (REC). The apparent activity is linearly proportional to the specific surface area (A_S), which is enlarged by RE dopants as a consequence of surface stabilization. To decouple the effect of each RE dopant on the surface inherent activity, we set A_S of REC to be almost constant by adjusting the pH during synthesis. In this case, however, pure ceria shows higher activity than any REC. We therefore conclude that although the RE dopants have lower intrinsic activity than that of Ce, they have an important effect of increasing A_S to a level that pure ceria can never attain synthetically, thereby increasing their catalytic activity.