The iron species in the lower valence state in steel slag are capable of thermochemical H2O splitting and producing hydrogen energy. The defining of the slag reactivity with varying particle size distributions was put forward. The results showed the hydrogen generation rate was primarily dependent on temperature and particle size. The kinetics analysis revealed a typical self-retardation reaction behavior for the fine slag, due to the clogging of macropores upon oxidation. In comparison, the reaction kinetics of the coarser slag were described well by three-dimension diffusion model and exhibited better resistance against the rate decline. The coarser slag had much more grain junctions and fewer fraction of RO phases distributed at the particle surface. Due to the lack of free surfaces, the volume expansion derived from the oxidation of RO phase would cause larger mismatch stresses at grain boundaries, and further trigger the cracks formation. (C) 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.