A rapid and facile microwave-assisted hydrothermal route has been developed to synthesize MnO2 nanocrystals with exquisitely tuned structures/morphologies by reducing potassium permanganate in the presence of hydrochloric acid. delta-MnO2 microspheres, alpha-MnO2 nanorods, gamma-MnO2 nanosheets and beta-MnO2 octahedrons have been obtained with 0.05, 0.10, 0.25 and 0.75 mL HCl (37 wt%), respectively. More importantly, gamma-MnO2 nanosheets and beta-MnO2 octahedrons were prepared for the first time by a one-step microwave-assisted method. The electrocatalytic activities of the synthesized MnO2 in KOH solution have been determined by cyclic voltammetry and the results show that all manganese dioxides can catalyze the oxygen reduction reaction (ORR) in alkaline medium with different catalytic activities. alpha-MnO2 nanorods appear to hold the highest catalytic activity due to their crystal phase/morphology with appropriate oxygen adsorption mode. (C) 2013 Elsevier Ltd. All rights reserved.