Reduction swelling always takes place during gas-based direct reduction of Hongge vanadium titanomagnetite-oxidized pellet (HVTMP) with hydrogen-rich gases and leads to discontinuous operation in shaft furnace smelting process. As an integral part of developing a novel clean smelting process of HVTM, the effect of reducing conditions on swelling behavior and mechanism of HVTMP is conducted with hydrogen-rich gases of dry pulverized coal gasification (DPCG), water-coal slurry gasification (WCSG), Midrex, and HYL-Ill. The results show that with the increase of reduction temperature, the reduction swelling index increases with all gases. The reduction swelling index is found to decrease with the increase of hydrogen content and in the order of DPCG> WCSG > Midrex > HYL-III at constant reduction temperature. The mechanism study shows that reduction swelling is caused by metallic iron whiskers which formed during the reduction process. The reduction temperature and hydrogen content change the amount and size of metallic iron whiskers, which affect reduction swelling. From the results of scanning electron microscope (SEM), a conceptual diagram is illustrated to describe the effect of reducing conditions on the growth of metallic iron whiskers. This study aims to provide both theoretical and technical basis for the effective utilization of HVTM using in shaft furnace. CO (C) 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.