The detailed reaction mechanism with microstructural changes of iron ore fines in H-2 atmosphere has been investigated using a high temperature drop tube furnace. With the model free method, the activation energy of 281 kJ/mol was obtained, indicating that solid-state diffusion of ions in lattice matrix of iron ore particle was the rate-controlling step. The analysis was therefore extended to obtain the microstructural information of the reduction product. It is found that dense product layers were formed and solid-state diffusion of iron and oxygen ions played a key role in the reduction process. The structure evolution of iron ore particles during the transformation from hematite to magnetite is the same as that from magnetite to wustite, while the transformation from wustite to metallic iron is clearly different. The microstructural changes and the elementary reaction steps during the two types of transformation are obtained and characterized. (C) 2019 Elsevier B.V. All rights reserved.