Bi2O3 nanomaterials with a diversity of morphologies, such as nanoparticles, nanorods and nanoplates, were controllably synthesized successfully by a reverse microemulsion route. X-ray diffraction (XRD), field-emission scanning electron microscope (FE-SEM), and thermogravimetry differential thermal analysis (TG-DTA) were employed to characterize the obtained products. It was found that the size and morphology of bismuth oxide were affected mainly by water content (W0), and the shape of Bi2O3 changed with different W0. The common role of surfactant adsorbing on the surface of the nanoparticles led to the formation of Bi2O3 nanomaterials with specific morphologies. The possible formation mechanisms of these nanostructures were also discussed. With calcination temperature increasing from 275 °C to 350°C, β-Bi2O3 has transformed into α-Bi2O3 completely.