Manganese oxides show huge structural flexibility and appear in various crystallographic polymorphs. Hence, morphological and phase control of desired manganese oxide nanostructures could enable their properties to be tuned with a greater versatility, and endow them with potential applications. Herein, we report a simple hydrothermal route for the synthesis of various MnO2 nanostructures using Mn3O4 powder as raw material. The obtained products were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Results show that in H2SO4 solution, urchin-like γ-MnO2 nanostructures and single-crystal β-MnO2 nanorods are obtained at 80 and 180 ℃, respectively. In addition, MnOOH nanowires were obtained in a dilute acid solution. The influence of synthetic parameters including temperature, acidity, and reaction time are discussed. The γ-MnO2 intermediate might play an important role in the formation of nanorods. The evolution of phases and morphologies in the reaction process suggested the anisotropic crystal growth for the formation of nanostructures under acidic conditions.