The rational design of highly efficient electrocatalysts to generate hydrogen by catalyzing hydrogen evolution reaction still remains a challenge. Herein, we report a simple strategy to significantly enhance the catalytic activities of NiCo2O4 nanowire arrays by simply tuning the amount of oxygen vacancies. Remarkably, the oxygen-deficient NiCo2O4 catalysts obtained in Ar environment show significantly improved catalytic activities toward hydrogen evolution reaction with the requirement of 104 mV overpotential to afford 10 mA cm(-2), 122 mV less than that for air-sintered NiCo2O4 (226 mV). Moreover, such catalysts also exhibit superior long-term durability for 24 h at 100 mA cm(-2). The present study further promotes the application of NiCo2O4 in other energy storage and conversion system. (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.