A hierarchical MoS2/MoO2 nanostructure is presented as a highly efficient catalyst for the hydrogen evolution reaction (HER), which is prepared via a facile hydrothermal route in the absence of templates. The influence of various Mo sources has been studied and the roles of MoS2 shell and MoO2 core are discussed. It is found that the MoS2/MoO2 nanostructures derived from the MoO2 nanobelts exhibit the best HER activity, and the HER performance is strongly affected by the relative contents of MoS2 shell and MoO2 core, which correspond to the mass transport ability and the electron transfer ability, respectively. (C) 2017 Elsevier B.V. All rights reserved.