A new process of solid state synthesis of BaFe12O19 with the assistance of mechanochemical hydration of Fe2O3 under planetary mill treatment is proposed. The increased surface polarity of hydrated Fe,O, induced coagglomeration with Ba(OH)(2) . H2O in less-hydrophilic organic liquids. Concurrent progress in size reduction enabled elaboration of micro-coagglomerates less than 1 mu m in size. The organic liquid media with adequate specific capacity and low viscosity resulted in enhanced reactivity and led BaFe12O19 powders with excellent magnetic properties and sufficient sinterability without irregular grain growth.