Hydrogen evolution reaction (HER) plays a critical role in electrocatalysis, and developing highly active, cheap and stable Pt-free catalysts for HER in alkaline media is imperative for conversion of renewable energy into hydrogen fuels via photo/electrochemical water splitting. Herein, we report a facile strategy to fabricate a high-performance Ru-NMCNs-T electrocatalyst (T is the annealing temperature) for HER, which consist of both Ru nanoparticles and single Ru atoms well dispersed on nitrogen-doped mesoporous carbon nanospheres (NMCNs). Ru-NMCNs-500 exhibited the best HER performance in the series. To achieve a current density of 10 mA cm(-2 )in 1 M KOH, it only needs an overpotential of 28 mV with a small Tafel slope of 35.2 mA dec(-1), superior to the commercial Pt/C catalyst. It also exhibited exceedingly improved long-term stability than Pt/C. This study can open a new avenue for preparing Ru-based catalysts toward HER under extreme alkaline conditions. (C) 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.