Ultrafine Fe3O4 powder was successfully synthesized via a novel ageing process from a precursor FeO(OH), which was the hydrolysate of FeCl3 in the urea solution. The structure of as-synthesized powder was characterized by X-ray diffraction (XRD), and the morphology of these nanoparticles was investigated using a transmission electron microscope (TEM). Pure phase Fe3O4 was obtained and the mean diameter of these nanoparticles was about 21 nm. Furthermore, the study indicated that the precursor FeO(OH) played an important role in the formation of Fe3O4 nanoparticles. The mechanism was also discussed. Ultrafine Fe3O4 powder was successfully synthesized via a novel aging process from a precursor FeO (OH), which was the hydrolysate of FeCl3 in the urea solution. The structure of as-synthesized powder was characterized by X-ray diffraction (XRD), and the morphology of these nanoparticles was investigated using the transmission electron microscope (TEM). Pure phase Fe3O4 was obtained and the mean diameter of these nanoparticles was about 21 nm. Further, the study showed that the precursor FeO (OH) played an important role in the formation of Fe3O4 nanoparticles. The mechanism was also discussed.