The Magnetite (Fe3O4) nanoparticles have been successfully synthesized through a solvothermal route by using FeCl36H2O and NH4HCO3 as the starting materials. The as-prepared products are characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), superconducting quantum interference device magnetometer (SQUID) and Brunauer-Emmett-Teller (BET). The uniform size of the Fe3O4 nanoparticles can be well controlled from 10 to 90 nm by changing the surfactants or the inorganic salts in the solvothermal process. The experiment results reveal that the magnetic properties of magnetite nanoparticles can be tuned by changing the particles size. In addition, the solvents in this reaction system have an important influence on the composition and morphology of the final products.