Iron was oxidized in the Fe-Ba(OH)(2)-NaOH-H2O-O-2 system at elevated temperatures. Fe3O4 and alpha-FeOOH were formed in the absence of oxygen. On the other hand, hexagonal plate-like particles of BaFe12O19, 1.0-4.3 mu m diameter, were formed as single phase by the oxidation of iron in Ba(OH)(2) and NaOH mixed solutions at 250-300 degrees C and above 1.0 MPa oxygen. The diameter of the particle significantly depended on the NaOH concentration. Additives such as CO32-, SO42- and CH3COO- promoted the grain growth of BaFe12O19.