Abstract: The nanocrystalline La_2O_3 were prepared by the combustion method, in which the lanthanum nitrate was conducted as oxidant, ammonium citrate as fuel and a little urea as initiating agent. The influences of the factors on product, including calcinating temperature, stoichiometry ratio between materials and release gases were studied. The thixotropic paste complex LaCit was heated and burnt at a temperature of 150 ℃, and then the gray precursor was obtained. The nanocrystalline La_2O_3 was prepared by calcining the precursor at 700 ℃ for 2 h. Then the samples were characterized by XRD, TEM and SEM. The results indicate that the morphology of nanocrystalline La_2O_3 is clarity-rectangled flakes on an average length of 100-150 nm, the width of 40-80 nm and the thichness of 20 nm. The yield of nanocrystalline La_2O_3 is 85;-87; with narrow size distribution and good dispersion of particles.