In this paper, monodisperse Fe 3 O 4 nanoparticles with single crystalline structure were synthesized via a facile environment-friendly method. And the size of the nanoparticles ranges from 10nm to 15nm. As-synthesized Fe 3 O 4 were characterized by X-ray diffraction instrument (XRD), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared (FT-IR) spectrometer and field emission transmission electron microscope (FE-TEM). The effect of tartaric acid (TA) amount on products was investigated by XRD and TEM. The results indicated that TA could commendably modulate the crystalline phase, morphology and size of nanometer Fe 3 O 4. A possible generated mechanism of Fe 3 O 4 crystals was proposed in virtue of UV–vis absorption spectra. Besides, the magnetic properties of as-synthesized Fe 3 O 4 were detected.