Monodispersed ca. 30 nm magnetite nanoparticles were synthesized in a 20/80 ethanol-water solution under ultrasonic irradiation. Both the ethanol-water solution and ultrasonic irradiation were important for the synthesis of the monodispersed magnetite nanoparticles. The ethanol-water solvent improved the monodispersibility of the magnetite particles, on the other hand, limited the formation of magnetite under mechanical stirring. The 100% cubic magnetite particles were successfully synthesized in ethanol-water solution under ultrasonic irradiation.