[en] The cerium dioxide nanoparticle (CeO₂ NP) (122 nm) aggregation process was investigated by nanoparticle tracking analysis (NTA), and the results were compared with those of dynamic light scattering (DLS). Unlike descriptions based on classical aggregation kinetics theory, the size distributions obtained by NTA and DLS show that aggregation of NPs in an aquatic environment is a complicated process and highly dependent on the particle number concentration, which is information that has been rarely presented in the literature. In particular, not all the particles were aggregated and there still remained some small CeO₂ NPs (< 400 nm), which has potential adverse environmental risks to the ecosystem and public health. Furthermore, the aggregate sizes are far smaller than previously shown in aggregation kinetics experiments. In addition, our findings also indicate that NTA can measure samples with mass concentrations ranging from 0.1 to 100 mg/L without dilution. In contrast, DLS cannot provide accurate information about aggregation kinetics when the mass concentration of CeO₂ NPs is lower than 1 mg/L (2.02 × 10⁸ ± 1.66 × 10⁷ by NTA data). Consequently, the classical aggregation kinetics experiment using DLS cannot accurately reflect the fate of NPs in aquatic environment, especially the heteroaggregation of NPs in complex water phase, and these results further demonstrate that the toxicity evaluation of NPs concerning different sizes measured by DLS need more rigorously experimental design. Therefore, our findings provide the first quantitative attempt to explore the experimental conditions and analytical methods for further study of aggregation kinetics.