Magnetite (Fe3O4) and Zn0.5Fe0.5Fe2O4 nanoparticles (NPs) were synthesized via sonochemical coprecipitation reactions with moderate ultrasound irradiation. The structure and crystalline/particle sizes of the synthesized NPs were determined by X-ray powder diffraction (XRD) and transmission electron microscopy (TEM), and the magnetic properties were measured by vibrating sample magnetometry (VSM). The results show that NPs of smaller size and narrower size distribution can be synthesized via coupling of moderate ultrasound irradiation in coprecipitation reactions. In addition, magnetic NPs prepared in the presence of ultrasonic irradiation exhibit improved magnetic properties.