Abstract: <P><FONT face=Verdana>Nano-sized lanthanum oxide was prepared with LaCl<SUB>3</SUB> and CO(NH<SUB>2</SUB>)<SUB>2</SUB> as raw materials by hydrolyzation of urea.The product was characterized by XRD,SEM,and DTA-TG.The influence of the ratio of reactants concentration,ultrasonic time,calcining temperature,and the dosage of bromide cetyltrimethylammonium (CTMAB) and (NH<SUB>4</SUB>)<SUB>2</SUB>SO<SUB>4</SUB> on the particle size of La<SUB>2</SUB>O<SUB>3</SUB> was studied,and the impact of the cluster appeared in synthesizing,drying and calcining the precursor on La<SUB>2</SUB>O<SUB>3</SUB> particle size had been effectively solved.Results showed that nano-sized La<SUB>2</SUB>O<SUB>3</SUB> with average particle size of 15 nm was prepared under the appropriate conditions of concentration of CO(NH<SUB>2</SUB>)<SUB>2</SUB>,ultrasonic time,calcining temperature,and the dosage of CTMAB and (NH<SUB>4</SUB>)<SUB>2</SUB>SO<SUB>4</SUB>.XRD result showed that the prepared La<SUB>2</SUB>O<SUB>3</SUB> belongs to hexagonal crystal system.<BR><BR></FONT></P>