Cerium oxalate nanoparticles were synthesized from the coupling route of microemulsion with homogeneous precipitation; the average size of nanoparticles was measured by BI-90 particle size analyzer. To make clear the influence of the thermodynamic stability of microemulsion used on the size of nanoparticles, the variation of the standard free energy of cosurfactant transfer from the continuous phase to the interface of microemulsion (ΔG c - > i ) was calculated by the dilution method. The results showed the variation of ΔG c - > i had complex effects on the average size of cerium oxalate nanoparticles; the reaction temperature and the sorts of surfactants also influenced the average size of nanoparticles.