The in situ electrochemical surface oxidation and associated disordered phenomenon under anodic condition are closely correlated with the electrocatalytic activities of cobalt-based compounds towards oxygen evolution reaction (OER). Herein, we report the interesting observations on this phenomenon in electrocatalytic systems of the cobalt tungstate (CoWO4) nanostructures and demonstrate the in situ oxidation and self-activation-induced amorphous reactive state determine their OER performances. The CoWO4 nanoflakes (NFs) with poor crystallinity are easily oxidized to composite-type CoOx/WOx and exhibit the best OER performance. To obtain a catalytic current density of 10 mA cm(-2), the CoWO4 NFs afford an overpotential as low as 436 my in an alkaline medium, which compare favorably to most previously reported Co-containing OER elecrocatalysts; Whereas the catalytic current density of the crystalline CoWO4 nanorods (NRs) and CoWO4 nanocuboids (NCs) cannot reach 10 mA cm(-2) even at the high potential of 1.90 V (vs. RHE). The in-depth mechanistic investigations demonstrate the excellent electrochemical OER behaviors of CoWO4 NFs are mainly originated from the in situ oxidation induced local structure change and amorphization. (C) 2016 Elsevier Ltd. All rights reserved.