High-density cocoon-like molybdenum sulfide (MoS2) nanostructures have been fabricated by thermal oxidation of a metallic Mo foil, followed by a simple sulfurization process under hydrothermal conditions. The MoS2 layer thickness is determined by that of the preformed oxide layer on Mo foils which can be readily tuned by thermal oxidation duration. The morphology and microstructure of the as-fabricated MoS2 nano-cocoons are investigated in detail by SEM, XRD, Raman spectroscopy, and TEM. The results show that the cocoons consist of many vertically aligned ultrathin MoS2 nanosheets with preferentially exposed edges, and that the MoS2 layer is intimately bonded to the underneath Mo substrate for samples having a small layer thickness. Electrochemical tests demonstrate that all MoS2-Mo cathodes exhibit high electrocatalytic activities, small Tafel slopes and good long-term stability for the hydrogen evolution reaction (HER). The outstanding HER performance can be attributed to the existence of abundant electrocatalytically active edge sites and structural defects, and to the intimate electrical contact between MoS2 and metallic Mo and the bind-free nature of the electrode, which facilitate electron transfer. Given the high electrocatalytic performance and the easy fabrication procedure, Mo supported cocoon-like MoS2 holds substantial promise to substitute platinum for use to catalyze HER in water electrolyzers. (C) 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.