Nano-sized crystalline γ-Fe2O3 (particle size ca. 7nm) was synthesized by mild oxidation of iron micelle obtained from Fe(CO)5. Heat treatment at 400°C in vacuum was effective to remove a large amount of organic impurities, the weight of which was over 40wt.% in the as-prepared solid phase. Significant suppression of the spinel-rocksalt transformation was confirmed as the major nano-sized effect and led to the reversible topochemical lithium interaction with the corresponding lattice volume expansion.