MnO2 nanostructures with different crystallographic structure and morphologies have been synthesized through a facile template-free hydrothermal method. The growth mechanisms of these MnO2 nanostructures have been discussed. At 140 degrees C, slim solid alpha-MnO2 nanorods are obtained. At 160 degrees C, the crystal phase begins to transform from alpha-MnO2 to beta-MnO2. And at 180 degrees C, uniformly hollow beta-MnO2 nanorods are formed. The electrocatalytic properties for oxygen evolution reaction (OER) of these as-prepared MnO2 samples have been systematically studied in alkaline media. It is obvious that the structure-function relationship of MnO2 samples for OER activity is dependent on the crystallographic structure and morphology, following the order alpha-MnO2 > alpha-MnO2/beta-MnO2 > broken beta-MnO2 > beta-MnO2. The best OER electrocatalyst is alpha-MnO2, with an overpotential of 0.45 V (at j = 10 mA cm(-2)). The enhanced OER activity is owing to not only the highest BET surface area (45.6 m(2) g(-1)), but also the largest electrochemical active surface area (C-dl = 3225.0 mu F cm(-2)). In addition, the intrinsic activity of each Mn atom in alpha-MnO2 (TOF = 0.0056 s(-1) at eta = 0.45 V) is larger than that of beta-MnO2 (TOF = 0.0003 s(-1) at eta = 0.45 V). (C) 2015 The Electrochemical Society.