The reduction of magnetite-based iron ore fines in a hydrogen-induced fluidized bed becomes an attractive fossil-free ironmaking route. Our previous study showed that a prior oxidation treatment of magnetite was helpful to improve its fluidization and reduction behavior. However, the underlying oxidation mechanisms of magnetite ore fines remained unclear and required further investigations. In this study, two magnetite ore brands were analyzed via in situ high-temperature X-ray diffraction (HT-XRD) during oxidation, to investigate the thermal transformation of Fe3O4 to α-Fe2O3 at crystal scale. The lattice constants and crystallite sizes of both phases and oxidation degree were evaluated at different temperatures based on the HT-XRD patterns. The lattice constants of Fe3O4 and α-Fe2O3 increased with an increase in temperature due to the thermal expansion and can be successfully fitted with temperature by second-order polynomials. With Fe3O4 being oxidized into Fe2O3, the Fe2O3 crystallite grew and showed a certain growth habit. The Fe2O3 crystallite grew faster along the a/b axis than the c axis. The oxidation kinetics followed the parabolic law as shown by the sigmoid-shaped oxidation degree curve, suggesting that the solid diffusion of ions was the rate-limiting step.