Barium ferrite platelet-shaped particles have been prepared with a diameter of 0.1 microns, which is an order of magnitude smaller than those obtained by conventional sintering and regrinding techniques. The preparation technique used has been optimized to produce the complete ferrite with σs=68 emu/g. Due to the small particle size, extremely high intrinsic coercive forces have been obtained, (Hc=5350 Oe), in good agreement with the value calculated from the Stoner-Wohlfarth (SW) coherent rotating model for single-domain noninteracting particles when shape and crystal anisotropy are both taken into account. Furthermore, the measured hysteresis loop for an unoriented sample agrees well with the theoretical SW loop. Similar hysteretic magnetization properties have been obtained in strontium ferrite. The single-domain nature of these powders is also demonstrated by their temperature dependence of coercive force where little change is obtained over the range −200° to +150°C. An estimate of the relative interaction field magnitude for the small particles of barium ferrite has been obtained by comparing its initial anhysteretic susceptibility with that of an acicular iron oxide powder.