Abstract: Ultrasonic technology was introduced in the preparation of magnetic particles in order to produce small size and homogeneous magnetic particles with uniform coating and steady magnetic fluid.The influence of operation power of ultrasonic wave on the size of magnetic particles and magnetic property was investigated in different preparation stages.The results indicate that the Fe3O4 magnetic particles produced with ultrasonic technology are better than those without ultrasonic technology in the process of co-precipitation,and the magnetic particles are uniform,small and not reunited easily.A steady coating was formed when the ultrasonic power was 40 W in coating process.The size of Fe3O4 magnetic particles is about 10 nm under optimization condition.