Heterostructured electrocatalysts with multiple active components are expected to synchronously address the two elementary steps in the hydrogen evolution reaction (HER), which require varied hydrogen-binding strength on the catalyst surface. Herein, electrospinning followed by a pyrolysis is introduced to design Fe3C-Mo2C/nitrogen-doped carbon (Fe3C-Mo2C/NC) hetero-nanofibers (HNFs) with tunable composition, leading to abundant Fe3C-Mo2C hetero-interfaces for synergy in electrocatalysis. Owing to the strong hydrogen binding on Mo2C and the relatively weak one on Fe3C, the hetero-interfaces of Fe3C-Mo2C remarkably promote HER kinetics and intrinsic activity. Additionally, the loose and porous N-doped carbon matrix, as a result of Fe-catalyzed carbonization, ensures the fast transport of electrolytes and electrons, thus minimizing diffusion limitation. As expected, the optimized Fe3C-Mo2C/NC HNFs afforded a low overpotential of 116 mV at a current density of -10 mA cm(-2) and striking kinetics metrics (onset overpotential: 42 mV, Tafel slope: 43 mVdec(-1)) in 0.5m H2SO4, outperforming most recently reported noble-metal-free electrocatalysts.