Abstract: In nuclear fuel reprocessing, plutonium oxalate precipitation is usually prepared by oxalic acid and then further calcined to obtain plutonium oxide particles, which are used to manufacture MOX fuel. Therefore, it is of practical significance to master the thermal decomposition mechanism of plutonium oxalate and control the morphology and particle size of plutonium oxide products. In this paper, cerium oxalate and uranium oxalate were focused. The thermal decomposition reaction of cerium oxalate and uranium oxalate was systematically investigated by thermogravimetric analysis/differential scanning calorimetry(TG/DSC) and X-ray diffraction(XRD). Cerium oxalate and uranium oxalate with different particle sizes were prepared. Samples before and after calcination are detected by laser particle sizer and scanning electron microscopy(SEM) to characterize the particle size, aggregation and morphology. The results show that calcination decomposition leads to the regular decrease of particle size, but the morphology is retained.