Monodisperse Fe3O4 and FeO nanocrystals (NCs) with different sizes (from 10 nm to 50 nm) and different shapes (cube, sphere, and ellipsoid) were synthesized by simply adjusting reaction temperature or molar ratio of Fe/oleic acid (OA) during the decomposition of FeO(OH) in noncoordinating solvent. The concentration of OA affected the nucleation and growth of NCs by improving the chemical reaction driving force during the syntheses of different types of iron oxide NCs. It has been found that the reaction temperature influenced the reaction activity between FeO(OH) and OA. The structure of Fe oleate complexes was studied using Fourier transform infrared spectroscopy (FTIR). X-ray diffraction (XRD), Raman spectroscopy, and transmission electron microscopy (TEM) were used for structural and chemical characterization of as-prepared iron oxide NCs. (C) 2008 Elsevier Inc. All Fights reserved