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°C, slim solid α-MnO2 nanorods are obtained. At 160°C, the crystal phase begins to transform from α-MnO2 to β-MnO2. And at 180°C, uniformly hollow β-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 α-MnO2 > α-MnO2/β-MnO2 > broken β-MnO2 > β-MnO2. The best OER electrocatalyst is α-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 m2 g-1), but also the largest electrochemical active surface area (Cdl = 3225.0 μF cm-2). In addition, the intrinsic activity of each Mn atom in α-MnO2 (TOF = 0.0056 s-1 at η = 0.45 V) is larger than that of β-MnO2 (TOF = 0.0003 s-1 at η = 0.45 V).