Matastable phase of iron (III) oxide, γ-Fe2O3, transforms to the stable phase, α-Fe2O3, by either isothermal wet ball-milling or vacuum hot-pressing. From kinetic analysis of these processes, it was found that application of external force makes an alternative  mechanism of the transformation operative. Decrease in the activation energy with increasing applied external force suggests that mechanical force elevates the potential energy of the reactant particles by enhancing strains or defects in the crystal. Comparing the results to those on the mechanochemical transformation of PbO, reported previously by the authors, reveals that mechanical force acts on the polymorphic transformation at least by two different ways depending on the nature of the material, although the mechanism of transmission of the external force to the particles remains almost unchanged irrespective of the material.