Iron oxide (Fe2O3) nanoparticles of distinct polymorphs (i.e., alpha and amorphous phases) were prepared in montmorillonite (MT) particularly with three types of methods (i.e., calcining, hydrothermal reaction, and hydrolysis) and used as photocatalysts for the oxidation of water to O-2. Although alpha-Fe2O3 has usually been applied to a photocatalysis system [employing the saturated loading of alpha-Fe2O3 (25 wt%) in MT], this study first showed that amorphous Fe2O3 has similar photocatalysis ability to alpha-Fe2O3 in terms of water oxidation. Moreover, irrespective of the types of polymorphs, their photocatalyses were dependent on the loading amount of Fe2O3 in MT; that is, the O-2 amount increased with the amount of the photocatalyst employed until the loading amount reached ca. 3 wt%, and decreased in the larger loading amount. In this article, the reason why such an optimal photocatalysis condition appears was considered in terms of kinetic aspects.