Fe3O4 nanoparticles were synthesized for the first time via an ammonia vapor diffusion process under ultrasonic irradiation. The possible ultrasound mechanism and the process of ammonia diffusion and reaction has been deduced. Influences of experiment parameters such as reaction time, temperature on the morphology, and size of products were also investigated in detail. Morphology and magnetic properties of products were carefully studied by Fourier transform infrared spectra (FTIR), X-ray diffraction (XRD), Scanning electron microscopes (SEM), and vibrating sample magnetometer (VSM). The synthesized Fe3O4 nanoparticles possess superparamagnetic properties at room temperature, and the saturation magnetization is 70.5 emu/g.