The formation mechanism of the monodisperse pseudocubic hematite particles obtained in large quantities by aging a highly condensed ferric hydroxide gel at 100°C has been studied. It was found that the Fe(OH)3 gel precipitated first soon dissolved to form very fine fibrous β-FeOOH, and then α-Fe2O3 nuclei grew by the deposition of the solute with the dissolution of the β-FeOOH. Appreciable nucleation of α-Fe2O3 already took place during the preparation of Fe(OH)3 at room temperature, as well as in the early stage of aging at 100°C. The nucleation was enhanced with a decrease in the excess concentration of ferric ions or an increase in the initial pH, yielding a product of much smaller size. It was elucidated by the increase in concentration of Fe(OH)+2 complex as a precursor of hematite with increasing pH. Thus, the initial drop of pH associated with the phase transformation of Fe(OH)3 into β-FeOOH in a given system was thought to contribute to the monodispersed particle formation of hematite by preventing nucleation during the following growth period. The pseadocubic crystal habit of the hematite particles bound by (012) faces was ascribed to the specific adsorption of chloride ions and/or chloro ferric complexes to the (012) faces restraining the growth in the directions normal to the (012) faces. The polycrystalline structure of the pseudocubic particles was attributed to the growth process through the stacking of ultrafine crystallites epitaxially grown from their two-dimensional nuclei, but strictly restricted in their growth and mutual fusion by the adsorbed chloride ions and/or chloro ferric complexes. The monodispersity was degraded to a great extent by agitation during the growth of the hematite particles due to the secondary nucleation.