Fe_3O_4 nano particles were synthesized at a variety of initial [Fe^<2+>] (0.001〜0.1M) with ultrasonic treatment. Two types of sonication were used in this work; a traveling wave type reactor and a standing wave type reactor. At initial [Fe^<2+>]=0.01M, it took 12h to complete the reaction by mechanical stirring. On the other hand, it took only 1h by ultrasonic irradiation. We found that using ultrasonic irradiation effectively reduced the reaction time. Particle sizes synthesized by traveling wave were reduced by increasing the output power of ultrasonic. The cavities were thought to be nucleation sites, and the number of cavities was increased with an increase in the output power. Therefore, in the traveling wave process, it was possible to control particle sizes by changing output power. In the standing wave process, it was possible to synthesize Fe_3O_4 at initial [Fe^<2+>]=0.001M. This was possible by neither mechanical stirring nor traveling wave process. In the traveling wave process, intensive macroscopic streaming by direct stream made a decrease of super saturation and nucleation was inhibited. In the standing wave field, on the contrary, because the whole solution had moderate microscopic streaming, super saturation was not decreased and nucleation was issued. Therefore, the standing wave process was appropriated to a low concentration ([Fe^<2+>]=0.001M) reaction. However, particles size distribution by the standing wave process in the high concentration ([Fe^<2+>]=0.01〜0.1M) was larger. Therefore, the traveling wave process was better suited for high concentration because particle size distribution was smaller. We must select the reaction process depending on the purpose.
