<p>Cerium oxide (CeO₂) is a critical abrasive in chemical mechanical polishing (CMP) due to its chemical-mechanical reactivity and planarization capability. This study developed a controllable hydrothermal synthesis strategy for spherical CeO₂ abrasives and systematically evaluated their CMP performance on silicon wafers. Uniform CeO₂ particles with a fluorite structure were synthesized, as confirmed by scanning electron microscopy, X-ray diffraction, and high-resolution transmission electron microscopy analyses. Mechanistic studies reveal that CeO₂ formation occurred via nitrate-induced oxidation of Ce³⁺. The subsequent precipitation of CeO₂, coupled with acetate decomposition, thermodynamically drove the reaction forward under hydrothermal conditions. By adjusting water volume, the particle size was precisely tuned from 32 to 531 nm while maintaining excellent sphericity and dispersity. CMP tests showed that 343 nm CeO₂ achieved the highest material removal rate (73.7 nm·min⁻¹) with low surface roughness (Ra = 0.31 nm). Optimal polishing with 0.5 wt% CeO₂ achieved a balance between removal efficiency and surface quality, while higher concentrations led to residue defects. Moreover, the hydrothermally synthesized CeO₂ exhibited better CMP performance than commercial abrasives in both short- and long-term polishing, owing to its uniform morphology. Overall, this work demonstrates a simple, effective, and scalable strategy for synthesizing size-controlled CeO₂ abrasives with excellent performance in precision surface processing.</p>