Depending on whether to add oleic acid in the reaction system, monodisperse magnetite nanoparticles with hydrophobicity and hydrophilicity are prepared by one-step "polyolthermal reaction". Combined techniques of TEM, XRD, XPS and FTIR are used to characterize the obtained nanoparticles. The diameter of nanoparticles can be tuned easily by a simple change in the reaction time and the concentration of iron precursor. The method proposed here combines all the advantages of the high-temperature thermal decomposition method and the solvothermal process. Details about formation processes of magnetite nanoparticles are studied using UV-vis absorption spectrum. The result shows a different nucleation procedure and nucleation time for oil-soluble and water-soluble nanoparticles due to the presence of oleic acid. Magnetic properties of the prepared magnetite nanoparticles are also measured on vibrating sample magnetometer at room temperature. (c) 2013 Elsevier Ltd. All rights reserved.