The extremely dispersed Fe3O4powders were prepared by hydrothermal reaction of FeSO4·7H2O and NaOH with aid of ultrasonic treatment. Phase identification was accomplished by XRD, and micromorphology was observed by FE-SEM. The results show that under the condition of strongly alkaline environment, poorly crystallizedα-FeOOH phase is synthesized at 120 ℃ and the Fe2O3phase is synthesized at 140 ℃. When the hydrothermal temperature is over 160 ℃, Fe3O4is formed. The grain size of the prepared Fe3O4increases w...