Abstract: By studing the correlation between the sizes obtained from XRD and TEM measurementsand the specific surface areas determined by BET method, several commercial acicular γ-Fe₂O₃ were studied for their microstructure. It is shown that the sizes obtained from X-ray diffraction line broadening are ones of coherent domain in which X-ray can be reflected in Phase; the domain size of the oxysch sublatticc is avcrage Size of the microcrystal or mosaic crystallite within aticular particle. For the micropores or polycrystalline particle, the quasi-spherical microcrystals are lined togother to form a chain like-of-spheres structure; for the microfissuring or single crystal particle, the oriented quasi-cylinder masaic crystallites are bonded together to form a laemodipodiform structure.