Large-scale production of hydrogen fuels via electrochemical water splitting requires efficient and cost-effective electrocatalysts. Molybdenum carbide (Mo2C), as a promising substitute for expensive Pt-based catalysts, exhibits outstanding electrocatalytic performance toward hydrogen evolution reaction (HER). Herein, by combining theoretical studies and experimental measurements, we systematically investigate the surface stability and HER activity of Mo2C. We demonstrate that the Mo2C(100) surface is oxidized under HER turnover condition, and the HER activity shows a positive correlation with the oxidation degrees of this surface. The Mo2C(100) surface covered by a monolayer of oxygen atoms is verified as the most stable surface structure, which derives the optimum HER activity, and thus the surface chemisorbed oxygen atoms are identified as the active sites. This study provides new insights into developing efficient electrocatalysts for HER based on Mo2C. (C) 2020 Elsevier Inc. All rights reserved.