To reduce the carbon content in the direct reduction iron (DRI) obtained via the two-step fluidized bed reduction process, we investigated the fluidization/defluidization behavior of the carbon-coated iron ore under different gas compositions and temperatures. It was found that the critical carbon content value (C-critical) needed to prevent defluidization increases obviously with increasing fluidization temperature, but is little affected by the gas composition, while the stability of the deposited carbon is highly dependent on the gas composition, i.e. when the H-2 mole fraction is greater than a certain critical value, the deposited carbon is unstable and will be constantly consumed during reduction, leading to the defluidization. Through the process optimization, the carbon content in DRI decreases to <5 wt.%, which is also much lower than those reported values of 16.5-22.3 wt.% in literature. (C) 2016 Elsevier B.V. All rights reserved.