A two-stage, fluidized reduction route is proposed to synthesize iron nanoparticles (NPs), with the aim of enhancing the quality of fluidization and preventing sintering activity. At both low and high temperatures, the degree of metallization is approximately 80% due to the defluidization. Defluidization is mainly caused by the rapid sintering of the newly formed Fe NPs. The proposed two-stage fluidization approach successfully resolves the defluidization problem through the self-agglomeration of nanoparticles cultivated at low temperatures. These self-agglomerated NPs showed an improved resistance to sintering at high temperatures. The high-purity Fe NPs prepared by this approach exhibited excellent combustion activity, indicative of the potential as oxygen carriers in chemical looping combustion systems. (c) 2016 American Institute of Chemical Engineers AIChE J, 63: 459-468, 2017