Surface treatment of nano-crystalline gamma-Fe2O3 was examined to improve its electrochemical performances for lithium battery. Removal of residual phases on the surface by chemical oxidation with NO2BF4 was confirmed by TG measurement and FT-IR spectroscopy, and the process improved charge-discharge characteristics; higher capacity and better reversibility were obtained. The charge-discharge mechanism was studied by X-ray diffraction measurement and Mossbauer spectroscopy. The reversible charge-discharge reaction of the nano-size gamma-Fe2O3 was proceeded by a phase transition between the spinel and the disordered rock-salt type, which was previously claimed to be an irreversible process for the bulk materials. The nano-size effect of the phase transitions and the charge-discharge mechanism will be discussed. (c) 2007 Elsevier B.V. All rights reserved.