Exploring efficient noble-metal-free water-splitting electrocatalysts from earth-abundant elements is of great importance to realize wide applications in the generation of hydrogen fuel for clean energy. Here, a facile route is reported to synthesize ��-Fe3N single-phase nanoparticles by thermal ammonolysis of Fe precursors. The roles of nitrogen atoms in tailoring the hydrogen evolution reaction (HER) activities of ��-Fe3N have been systematically investigated. HER activity is enhanced by reducing the effective coordination number of nitrogen atoms from 2.61 to 1.67, where the standard coordination number in ��-Fe3N is 2. Density functional theory calculations reveal that the reduction of nitrogen content lowers the energy of Tafel process on the ((Formula presented.) 100)-FeN-exposed and (11 (Formula presented.) 0) N-exposed surfaces. Both surfaces are thermodynamically favored for the HER. Furthermore, the active sites of Tafel process change from the kinetically less favored hollow sites of Fe atoms to the kinetically more favored top site of N atoms and the bridge site of Fe atoms on both ((Formula presented.) 100)-FeN and (11 (Formula presented.) 0) N-exposed surfaces. The findings propose a novel strategy to enhance HER activity by using nitrogen deficiency, which is of great importance for the development of highly active transition metal based electrocatalysts.