Superparamagnetic Ba-hexaferrite, BaFe12O19, nanoparticles were prepared by reacting tetrahydroxoferrates (III), [Fe(OH)(4)]-(aq), and barium ions, Ba2+(aq), in an autoclave in the presence of a high concentration of hydroxide ions OH-. The synthesis temperature was between 80 degrees and 140 degrees C. The size and the morphology of the synthesized nanoparticles were confirmed by transmission electron microscopy. The crystal structure of the nanoparticles was characterized by X-ray diffraction. The powders synthesized above 120 degrees C exhibited a bimodal particle size distribution, while those synthesized below 120 degrees C showed a monomodal particle size distribution. The field-cooled/zero-field-cooled magnetization measurements showed typical superparamagnetic behavior. The superparamagnetic nanoparticles synthesized below 100 degrees C exhibited a disk-like shape, on average < 10 nm wide and 3 nm thick, with a room-temperature magnetization of approximately 3 emu/g at 1 T.