Cerium oxide (CeO₂) nanoparticles have been considered as excellent antioxidants and have become a focus of numerous studies. However, the mechanism behind the antioxidant role in the complex biological system has not been well-understood. In this work, direct evidence for the hydroxyl radical (•OH) scavenging activity of CeO₂ nanoparticles was established by a simple photometric system in vitro. When methyl violet (MV) reacted with hydroxyl radical, the absorbance change indicated the hydroxyl radical level. The presence of CeO₂ nanoparticles protected MV by competitively reacting with the •OH, so their hydroxyl radical scavenging activity was directly seen through the absorbance change. This activity was also proved to be size-dependent and was believed to have a close correlation with Ce³⁺ at the surface of the particles.