We report a simple solvothermal method for the synthesis of monodisperse magnetite nanoparticles with a controlled particle size within the range of 40 to 200 nm from available and inexpensive single iron precursor (FeCl3) and, as co-capping agents, sodium acetate and ethylene diamine. The particle size can be easily controlled by the reaction time of synthesis. Transmission electron microscopy, x-ray diffraction, Fourier transform infrared spectroscopy, and x-ray photoelectron spectroscopy techniques were used to investigate the obtained particles. The results revealed that the resulting iron oxide particles exhibit a single magnetite Fe(3)O(4)phase and high stability in air even for four months. The high air stability of these magnetite could be due to the surfactant capped on the particle surface. This method of synthesis has some advantages including simplicity, high product quality, and acceptable reproducibility. Therefore, the magnetite particles can be used for different applications such as in catalysis and drug delivery.