Monodisperse manganese oxide flowerlike nanostructures have been prepared facilely at low temperature and ambient atmosphere. The effect of the reaction time on the microstructure and morphology is observed systemically by transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM). Meanwhile, the possible formation mechanism of the flowerlike nanostructures has been proposed and discussed. It is also found that the reaction temperature has great influences on the morphology of these unique nanostructures. The results of nitrogen adsorption-desorption experiments and electrochemical measurements show that the product obtained at 40 °C for 8 h has large specific surface area, uniform pore size distribution, and excellent capacitance performance, which make it a potential supercapacitor electrode material.