The determination of active sites of materials is essential for the molecular design of high-performance catalysts. In this study, the first-principles method is applied to investigate the active sites of low-cost Ni metal-based electrocatalysts for hydrogen evolution reactions (HER), which is a promising alternative to expensive Pt metal-based catalysts. The adsorption of hydrogen on different sites of pristine and partially oxidized Ni(111) surface is investigated. All of the possible configurations have been systematically investigated here with the consideration of their Boltzmann distribution. Using the Gibbs free energy of intermediate H atoms (ΔGH*) as a descriptor, it is found that the ΔGH* increases with the increase of the coverage of oxygen atoms. The slightly oxidized surface Ni atoms are theoretically identified to be the best catalytic centers for the electrocatalytic HERs when the coverage of oxygen is considerably low. On the basis of the analyses of Bader charge distribution and density of states, our results reveal that the superior performance of the slightly oxidized surface Ni atoms can be ascribed to the optimal electronic properties.