Here, we provide a facile ultrasonic synthesis of amorphous Fe3O4 nanoparticles with diameters of about 5 nm using FeCl3 and the organic solvent of ethanolamine (ETA) at room condition. The intermediate of the ETA-Fe(II) complex produces Fe3O4 after hydrolysis treatment. The moderate reduction of ETA and ultrasound play an important role in the synthesis of superfine Fe3O4 particles with a very high specific surface area (270.94 m(2) g(-1)). The Fe3O4 nanoparticles were characterized by X-ray diffraction (XRD), scanning and transmission electron microscopy (SEM and TEM), high-resolution transmission electron microscopy (HRTEM), and ultraviolet-visible absorption spectroscopy (UV-vis). Fe3O4 as an electrode material was fabricated into supercapacitor and characterized by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and galvanostatic charge/discharge measurements. The as-synthesized Fe3O4 exhibits remarkable pseudocapacitive activities including high specific capacitance (335.5 F g(-1) at 0.4 A g(-1)), good rate capability (54.7 F g(-)(-1) at 10 A g(-1)), and excellent cycling stability. The novel synthetic route of Fe3O4 is a convenient and potential way for the secondary energy material which is expected to be applicable in the synthesis of other metal oxide nanopartides. (C) 2014 Elsevier B.V. All rights reserved.