Several shapes of BaSO4 particles were synthesized by the reaction of Ba(AOT)(2) microemulsion with NaAOT microemulsion containing SO42- at differing [Ba2+]/[SO42-] mole ratios. BaSOa nanofilaments were obtained under an excess of Ba2+ with the same mole ratio to that of Li and Mann (2000). A plasma replica SEM revealed that the nanofilaments were formed exactly in the microemulsion. Further excess Ba2+ resulted in the formation of short rags and thick spirals rather than nanofilaments. In contrast, fine particles were formed under a shortage of Ba2+. By addition of excess Ba2+ to the microemulsion after the generation of the fine particles under a shortage of Ball, whisker-like rods were obtained. These results suggest a new mechanism for formation of the whisker-like rods through an anisotropic aggregation of the fine particles.