Due to its poor electrical conductivity and finite exposed active sites, the development of high activity Co3O4 oxygen evolution reaction (OER) electrocatalysts remains a major challenge. Oxygen vacancies can enhance the electrical conductivity of electrocatalysts and reduce the adsorption energy of H2O molecules on surfaces, thereby significantly enhancing their electrocatalytic activity. Taking inspiration from this, we demonstrate a green and facile reduction strategy to prepare reduced Co3O4 nanosheet arrays (R-Co3O4 NSA) with large electrochemical surface area and rich in surface oxygen vacancies. Compared to pristine Co3O4 nanosheet arrays (P-Co3O4 NSA), R-Co3O4 NSA exhibits better OER performance, with a lower overpotential of 330 mV at a current density of 20 mA cm(-2) and a smaller Tafel slope of 72 mV dec(-1). Impressively, the excellent properties of R-Co3O4 NSA can rival to the state-of-the-art noble metal oxide electrocatalyst (IrO2). This strategy of defect-engineering offers a briefness and cost-effective means for the development of highly efficient OER systems. (C) 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.