M-type hexaferrites are one of the most important magnetic materials due to their applications as permanent magnets, magnetic recording media, microwave components and devices, etc. We report a study on the correlation between the synthesis procedure on the microstructure and the magnetic properties of BaFe₁₂O₁₉ nanopowders. These were synthesized by two modified co-precipitation methods: microemulsion co-precipitation and sonochemical co-precipitation (sonochemistry). We used a water-in-oil reverse microemulsion system with CTAB (24 wt.%) as a cationic surfactant, n-butanol (16 wt.%) as a co-surfactant, n-hexanol (20 wt.%) as a continuous oil phase, and an aqueous solution of metallic ions (40 wt.%) for the microemulsion method. In the case of sonochemical co-precipitation, high-power ultrasound stirring was applied during the co-precipitation process. The obtained precursors were subjected to high-temperature synthesis. The BaFe₁₂O₁₉ powders consisted of particles exhibiting an irregular shape between a sphere and a hexagonal platelet, as the process of formation of the platelet shape typical of the BaFe₁₂O₁₉ hexagonal structure was interrupted due to the small particle size. The average particle size of BaFe₁₂O₁₉ nanopowders is 140 nm and 86 nm for microemulsion and sonochemistry, respectively. The magnetic hysteresis loops were measured at room temperature in an applied magnetic field of 30 kOe. The sample obtained by microemulsion had a high coercivity value of 4 kOe, while the powder obtained by sonochemical co-precipitation was 140 Oe. The BaFe₁₂O₁₉ powders consisted of particles with a size below 150 nm and exhibited an irregular shape between a sphere and a hexagonal platelet, as the process of formation of the platelet shape typical of the BaFe₁₂O₁₉ hexagonal structure was interrupted due to the small particle size.