The facile hydrothermal method was used to synthesize Fe3O4 nanoparticles with an averagediameter of 11nm. The pure body-centered cubic (bcc)-Fe nanoparticles were prepared by reductionof Fe3O4 nanoparticles powder in H2 atmosphere. The structure, morphology and magnetic propertiesof the products were characterized by X-ray powder diffraction (XRD), transmission electronmicroscopy (TEM), thermogravimetric analysis-differential scanning calorimetry (TGA-DSC) andvibrating sample magnetometer (VSM). The results showed that the as-prepared Fe3O4 nanoparticleshad a relatively homogeneous size. The particle diameters became bigger with the increaseof reaction time. The growth kinetics of the Fe3O4 nanoparticles was also briefly discussed. Theproducts exhibited superparamagnetic properties at room temperature and the specific saturationmagnetization was dependent on the particle sizes.