Hausmannite (Mn3O4) nanoparticles were prepared by ultrasound in various media for the first time under ambient conditions without any additives. The synthesis was carried out by direct sonication of manganese acetate in water. The nanoparticles with high crystallinity and sizes less than 4 nm appeared after 3 min of sonication. Water in oil emulsion was selected as another medium for the synthesis. In this case, the water phase containing manganese acetate was dispersed into the oil phase. Two different types of oil were chosen as a medium for the synthesis paraffin as a mineral oil; olive and almond as vegetable oils. In all selected media, hausmannite was prepared with different sizes, shapes, and crystallinity. In the vegetable oils, the nanoparticles were larger than the nanoparticles prepared in water but the crystallinity was lower than in the water phase. In contrast, the crystallinity of the product in mineral oil was higher than in vegetable oil and lower than in water but the particle size in mineral oil was larger than the two other media. TEM (transmission electron microscopy) and XRD (X-ray diffraction) were used for characterizing the products. The XRD patterns confirmed that different media yielded to the same product, Mn3O4 nanoparticle. The TEM images showed that the size and shape of nanoparticles were different in the different media. In vegetable oil, nanoparticles had a spherical shape with maximum 7 nm in size. Using mineral oil, the shape of nanoparticles was diamond form and maximum 50 nm in size. In water phase, the size of nanoparticle is less than 4 nm with spherical shape. (C) 2010 Elsevier B.V. All rights reserved.