Pure Fe 3 O 4 nanocrystals with different sizes were synthesized via hydrothermal processing of an easily obtained, air-stable metallorganic molecular precursor (ferric acetylacetonate: Fe-(ACAC) 3 ) in 20vol.% hydrazine hydrate aqueous solution at 80–160°C for 0.5–12h. The phase, purity and size of the resultant products were characterized by powder X-ray diffraction (XRD), Raman spectroscopy, Fourier transform infrared spectroscopy (FTIR) and transmission electronic microscope (TEM), and the possible formation mechanism of Fe 3 O 4 was tentatively proposed. The magnetic properties of the products have also been studied.