Abstract: The precursor of lanthanum oxide with a low chlorine content was obtained by liquid precipitation with the assistance of microwave and ultrasonic. In this process, ammonium bicarbonate was taken as precipitant and lanthanum chloride solution as raw materials. Better optimum reaction parameters were determined by studying how the feeding mode, heating mode, reaction temperature, solution concentration, molar ratio of reactants and ultrasonic mode would affect the chlorine content of precipitate and the morphology of the precursor of lanthanum oxide. The results showed that a bar-like precursor of lanthanum oxide with a good dispersity, a smooth surface and a chloride content as low as 0.005 2 % could be prepared by liquid precipitation with the assistance of microwave and ultrasonic when the reaction temperature reaches 50 ℃ with parallel flowing mode used, the concentration of lanthanum chloride solution being 0.1 mol/L, the molar ratio of lanthanum chloride to ammonium bicarbonate 1:3.5, the ultrasonic power 500 W and the ultrasonic mode 1:1.