Polycrystallite fine particles and thin films of spinel ferrites ((Fe, M)3O4, M=Fe, Co, Ni, Mn, Zn, Ti, etc.) are synthesized directly from an aqueous solution at 3-100°C, utilizing Fe2+→Fe3+ oxidation. This low temperature process allows us to fabricate new ferrite film devices on substrates of such non-heat-resistant materials as plastics. It also allows us to immobilize bio-active molecules directly onto the surfaces of ferrite fine particles during their synthesis, which is promising for biomedical device applications. The fine particles and thin films, though prepared at the low temperatures, have saturation magnetization of magnitude favorably compared to that observed for bulk ferrite samples. The principle of the aqueous synthesis of ferrite is described.