The microstructure of the nanometer-sized manganic oxide (Mn2O3) has been investigated by the X-ray diffraction (XRD), transmission electron microscopy (TEM), and high-resolution electron microscopy (HREM). The experimental results suggest that annealing temperature and time, dripping speed of the NaOH aqueous solution, and pH value during aqueous solution chemical reaction have a great effect on the microstructure of the nanometer-sized Mn2O3. We found that the grain size of the nanometer-sized manganic oxide (Mn2O3) increases with the increase of annealing temperature and the decrease of the pH value of the aqueous solution. With change in the NaOH aqueous solution dripping speed, a large difference was observed in the TEM images of the nanometer-sized Mn2O3.