BaFe12O19 hexaferrite nanoparticles, containing cetyltrimethylammonium chloride (CTAC), n-hexanol, and cyclohexane, were synthesized by a reverse microemulsion technique with a combination of (NH4)(2)CO3 and NH3 center dot H2O as precipitator. Barium ferrite nanoparticles with 30 nm diameter and uniform flaky structure were proved to be single magnetic domains, which have magnetic properties comparable to some of the best ever reported for fine barium ferrite powders by chemical methods. Heat-treatment conditions can significantly influence the formation of pure BaFe12O19 hexaferrite phase, where quenching and nonprecalcination would produce intermediates of alpha-Fe2O3 and BaFe2O4, as detected by X-ray diffraction (XRD) and Fourier transform infrared (FT-IR) analyses, resulting in lower magnetic properties. High magnetocrystalline anisotropy constant K and energy barrier E-A calculated from Stoner-Wohlfarth theory may also account for the high coercivity for pure BaFe12O19. The variation of electrical conductivity during the formation and reaction of microemulsion droplets suggests nonpercolating microemulsion conducting systems. Transmission electron microscopic (TEM) images of the microemulsion droplets from a microemulsion system with R = V(water):V(oil) = 1:8 displayed microemulsion droplets about 100 nm, containing a barium ferrite precursor "core" of about 30 nm in size, with collision and coalescence being discovered.