Electrocatalytic nitrogen reduction reaction (NRR) at ambient conditions holds great promise for sustain-ably synthesizing ammonia (NH3), while developing highly-efficient, long-term stable, and inexpensive catalysts to activate the inert N≡ N bond is a key scientific issue. In this work, on the basis of the concept“N-heterocyclic carbenes (NHCs)”, we propose a carbon decorated graphitic-carbon nitride (C/g-C3N4) as novel metal-free NRR electrocatalyst by means of density functional theory (DFT) computations. Our re-sults reveal that the introduced C atom in g-C3N4 surface can be regarded as NHCs and catalytic sites for activating N≡ N bond, and are stabilized by the g-C3N4 substrate due to sterically disfavored dimeriza-tion. Especially, this NHCs-based heterogeneous catalysis can efficiently reduce the activated N2 molecule to NH3 with a low overpotential of 0.05 V via an enzymatic mechanism. Our work is the first report of NHCs-based electrocatalyst for N2 fixation, thus opening an alternative avenue for advancing sustainable NH3 production.