Abstract: The variations of magnetic coercivity (H_c) during the phase changes between Fe₃O₄ and γ-Fe₂O₃ for granular powders and continous thin films are investigated. Whether during the oxidation from Fe₃O₄ to γ-Fe₂O₃ or during the reduction from γ-Fe₂O₃ to Fe₃O₄, the increases in H_c of the spherical particles and the continous thin films are mainly caused by the stress anisotropy, and the decreases in H_c of the acicular particles are affected with the shape anisotropy related to the changes in particle size.