A membrane dispersion-based microreactor was used to prepare ceria nanoparticles using cerium nitrate and ammonia-water as the raw reagents., The traditional precipitation process makes it difficult to control crystal site in a stirred tank reactor; therefore, a new membrane dispersion microreactor was used, which was able to easily control the crystal size. Most importantly, the size of ceria nanoparticles was significantly reduced because of the enhanced mixing performance of the membrane dispersion microreactor: The crystal size of the prepared ceria in this new procedure reached 8.2 nm in comparison to similar to 16.7 nm from the traditional precipitation process under identical conditions. The effects of supersaturation, pH value, mixing intensity, and reaction temperatures, were also investigated in. detail. Therefore, ceria nanoparticles with average sizes of 7-12 nm can be controllably obtained. Furthermore, the reaction time to, reach 90% AO7 degradation rates was improved by similar to 89.2% using the ceria prepared in the membrane dispersion microreactor compared to that in a stirred tank reactor with an initial AO7 concentration of 60 ppm and H2O2 concentration of 80 mmol/L. To conclude, this study provides a size-controllable preparation method for ceria nanoparticles, particularly with smaller particle sizes and superior catalytic activity.