Powdery hausmannite (Mn3O4) is of interest in many industrial and technological applications. It is widely used as reactive catalysts, raw material of humidity sensors, and the cathode oxides of Li-ion secondary batteries. In this study, sub-micron and nano-meter sized Mn3O4 powders are prepared by an efficient method at room temperature. Mn(OH)2 nanocrystalsare commonly precipitated at first and then oxidized in the alkaline solution containing excess OH- anions. However, conventionally prepared Mn3O4 powders by the above process are ill-crystallized. To enhance the crystallinity of fabricated powders, CO3 2- anions are introduced into the process. The modified autoxidation method is practical to fabricate low-cost and high grade powders of Mn3O4. Advantages of the modified method are confirmed by both the electron micrographs and XRD patterns of synthesized powders. It is revealed that particle size of the products is in the sub-micron meter range, and the particle morphology can be adjusted by altering the precipitation sequence.