Cerium oxide superfine powder with average first grain size 140 nm and second gr ain size 630 nm was prepared by co-precipitation method, using industrial ceriu m carbonate as raw material and NH4HCO3 as precipitation agent. Characterist ic study was also made by TG-DTA, XRD and TEM. The result shows that cerium oxi de nanometer crystal can be prepared by using NH4HCO3 as precipitation agent , and the sediment can be completely decomposed in the temperature at 360 ℃. The pr epared cerium oxide superfin...