Crystal plane engineering is considered to be an effective means to improve the surface activity of catalysts recently. Nevertheless, study related to crystal plane dependent electrocatalytic activity for hydrogen evolution reaction (HER) is still scarce. Here, two NiS2 nanocrystals with different crystal planes ({1 1 1} planes and {1 0 0} planes) were designed and controllably prepared. It was demonstrated that the NiS2 nanocrystals with {1 1 1} planes showed an enhanced HER activity (an overpotential of 138 mV to achieve a current density of 10 mA/cm(2)) compared with the NiS2 nanocrystals with {1 0 0} planes (an overpotential of 302 mV to achieve a current density of 10 mA/cm(2)). Through firstprinciples calculation based on density functional theory, mechanism of the crystal plane dependent electrocatalytic activity was studied in detail. It was found that the {1 1 1} planes of NiS2 had much higher surface energy and better H adsorption capacity compared with that of {1 0 0} planes, which both contributed to the enhanced HER activity. The present study provides a typical case for crystal plane dependent HER electrocatalytic activity, which is of great significance for deep understanding the HER mechanism and accurate designing of high efficiency HER electrocatalysts. (C) 2019 Elsevier Inc. All rights reserved.