Supported Ni-based nanocatalysts have attracted much attention to replace noble metal catalysts (e.g., IrO2) for the oxygen evolution reaction (OER) due to their low costs. However, their low activity is the main hindrance for their use in the practical OER application. In this study, a Ni-based core-shell material (Ni@Ni-NC) is produced through the heat treatment of a mixture of urea and NiCl2 center dot(H2O)(6). Multiple analysis data reveal that the Ni@Ni-NC consists of a Ni nanoparticle core and several tens of nanometer-thick, N-doped carbon (NC) shell materials, in which atomically attached Ni-based species were homogeneously distributed. Ni@Ni-NC exhibits excellent electrocatalytic OER performance with over- and onset potentials of 371 mV and 1.51 V, respectively, which are better than those of commercial IrO2. As control samples, structural and electrochemical properties of various composites (Ni nanoparticles + N-doped graphene, Ni nanoparticles + C3N4, atomically dispersed Ni on a C3N4 surface) and acid-treated Ni@Ni-NC are investigated. These experiments reveal that the well-dispersed Ni-NC species and core-shell structures play pivotal roles in improving the electrocatalytic OER performance. Furthermore, density functional theory (DFT) calculations suggest the dual-site OER mechanism of the Ni-NC active species with a significantly low reaction barrier. The mechanisms for the formation of core-shell structures are studied with control samples, which are produced from different heating times, and DFT calculation suggested that the core/shell structure formation is attributed to the cohesive energy of the Ni particles and strong bonds between the Ni and NC supports. This work provides a facile strategy for designing supported Ni catalysts with core-shell architecture for electrocatalytic reactions and other advanced applications.