Strontium hexaferrite nanopowders were prepared by mechanochemical synthesis from strontium and iron oxides using a high energy ball mill after 50 hours of milling. The synthesis process was checked by X-Ray diffractograms on powders milled for different times. The magnetic properties of hexaferrite nanopowder, both compacted and dispersed in a nonmagnetic matrix were determined. Severe stresses and structural deformations were introduced by mechanical processing, but they were eliminated, to a great extent, by a suitable heat treatment of the milled powder at 1000 oC for one and a half hour. Coercivities as high as 6600 Oe and specific magnetization of 65 emu/g were found for annealed noninteracting nanopowders. Such values are very near to the theoretical values for strontium ferrite. The magnetic behavior of such powders can be rather well described by the coherent rotation model of Stoner-Wohlfarth for an assembly of single domain particles oriented at random. Sintered bodies of such powders produced magnets with a high HC of 4600 Oe, a Br of 2100 Gs and an energy product maxim of approximately 1.85 MGOe.