Development of non-noble-metal electrocatalysts with high activity and durability is a competitive strategy for the practical applications of water splitting which including hydrogen evolution reaction (HER) as well as oxygen evolution reaction (OER). In this work, high-quality CoS2 ultrathin nanosheets were synthesized through a simple and efficient hydrothermal method. After merging 10% wt Ni, the bimetal catalyst (Co0.9Ni0.1S2) displays excellent catalytic activity (10 mA cm(-2) at an overpotential of 156 mV and a small Tafel slope of 52 my dec(-1)) as well as stability (over 3000 cycles without obvious decay) in acid condition. Based on the results of Density Functional Theory (DFT) calculations, H+ adsorbed competition, hydrogen formation energetic and kinetics are proposed to help us understand the high efficient HER process over the Co0.9Ni0.1S2 (CNS) ultrathin nanosheets. (C) 2017 Elsevier Ltd. All rights reserved.