Birnessite flower-like and alpha-type tubular MnO2 nanostructures were selectively synthesized through simple decomposition of KMnO4 under hydrochloric acid condition by controlling reaction temperature using a microwave-assisted hydrothermal method. The as-prepared samples were characterized in detail by various techniques including X-ray diffraction, field emission scanning electron microscopy, transmission electron microscopy, high-resolution transmission electron microscopy, Fourier transform-infrared spectroscopy, and Raman scattering spectroscopy. While the growth of flower-like birnessite-MnO2 might follow a widely accepted Ostwald ripening process, we proposed a formation mechanism of the nanotubular alpha-MnO2 based on our evidence, which was assembly of nanorods through an "oriented attachment" process. (C) 2011 Elsevier Inc. All rights reserved.