Barium hexaferrite BaFe12O19 nanoparticles with a single-domain size were synthesized using a controlled hydrothermal process involving the LaMer-Dinger principle and the Ostwald ripening mechanism. Nanocrystalline particles of BaFe12O19 were obtained when the molar ratio of the precursor composition Ba(OH)(2)center dot 8H(2)O/gamma-Fe2O3 was 0.3 and the concentration of the suspension was about 1 wt%. The as-synthesized crystalline BaFe12O19 platelets approximately 50 nm in length and 5 nm in thickness exhibited a saturation magnetization of 40 Am-2/kg.