Due to the conflicting reports on the antioxidant activity of cerium oxide nanoparticles, much work has been done to explore the factors influencing the antioxidant activity of nano-CeO₂. However, most of the research was focused on external factors rather than the intrinsic properties such as the exposed crystal planes of the material itself. Here, we synthesized three kinds of nano-CeO₂ with different morphologies by a hydrothermal process. These materials showed antioxidant ability in the following order: nanoparticles < nanobars < nanowires. The probable reason for this distinction is due to the difference in their exposed crystal planes. Nanoparticles mainly expose the stable plane (111), while nanobars favor to expose the active plane (110) and the more active plane (100). Nanowires expose the same lattice planes as nanobars but show a higher ratio of (100)/(110) due to their longer and thinner structures. The vacancy formation energy of the (100) and (110) planes is lower than that of the (111) plane, and the Ce⁴⁺/Ce³⁺ recycle is easier for nanobars and nanowires than that for nanoparticles, which results in an improved antioxidant activity. Nanowires expose a higher ratio of (100)/(110), thus its antioxidant activity is better than that of nanobars. This result may be helpful for further understanding of the mechanism of the antioxidant activity of nano-CeO₂.