New electrocatalysts to replace noble metal catalysts for the hydrogen evolution reaction (HER) are highly desired to produce renewable and environmentally-friendly energy. In this work, nanostructured nickel phosphides supported on carbon nanospheres (CNSs) with different carbon content (Ni2P/CNSs-x, x = 10, 20, 40, 60) are synthesized by thermal decomposition using nickel acetylacetonate as nickel source and trioctylphosphine as phosphorus source in an oleylamine solution containing CNSs for the first time. The structure and morphology are characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), energy dispersive X-ray analysis (EDX), X-ray photoelectron spectroscopy (XPS), and N-2 adsorption-desorption. Then the electrocatalytic properties of as-synthesized Ni2P/CNSs-x for the HER are studied. In addition, the Ni/CNSs-20 hybrid is synthesized and the electrocatalytic properties are studied. The results show that all the Ni2P/CNSs hybrids exhibit higher catalytic activity than the Ni/CNSs-20 hybrid. The catalytic activity of the as-synthesized Ni2P/CNSs hybrid can be enhanced by changing the carbon content. The superior catalytic activity is attributed to the coupling effect between the Ni2P nanoparticles and CNSs, the electronic effect of Ni, the ensemble effect of P, the large surface area, and the high electron conductivity of CNSs. This study paves the way for the design of HER electrocatalysts with high performance and low-cost that can be employed under acid conditions. (C) 2015 Elsevier B.V. All rights reserved.