AbstractCeO₂ and CeO₂‐based materials are widely used as catalysts and catalyst supports for a variety of chemical reactions. The ability to form oxygen vacancies plays an important role in the catalytic activities in these materials. Therefore, revealing the reduction mechanism for CeO₂ is crucial to understanding the catalytic activities. In this study, shape‐controlled CeO₂ nanoparticles are fabricated and the distribution of surface oxygen vacancies on the (100) and (111) surfaces is systematically studied using scanning transmission electron microscopy and electron energy‐loss spectroscopy and the response to H₂ reduction treatment. It is successfully demonstrated that both catalytic activities and the ability to form oxygen vacancies are strongly dependent on the type of lattice planes. Moreover, the present results provide important insights into the reduction mechanism for CeO₂, in which bulk oxygen instead of the widely believed surface capping oxygen makes no small contribution to the initial reduction step.