The electrocatalytic nitrogen reduction reaction (NRR) has been regarded as a promising strategy for producing ammonia (NH 3 ) at ambient conditions. However, the development of the NRR is severely hindered by the difficult adsorption and activation of N 2 on the catalyst surface and the competitive hydrogen evolution reaction (HER) due to the lack of efficient NRR electrocatalysts. Herein, Mo 2 C-MoO 2 heterostructure quantum dots embedded in reduced graphene oxide (RGO) are proposed as efficient catalysts for the electrocatalytic NRR. The ultrasmall size of the quantum dot is beneficial for exposing the active sites for the NRR, and the synergetic effect of Mo 2 C and MoO 2 can promote N 2 adsorption and activation and suppress the competitive HER simultaneously. As a result, a well-balanced NRR performance is achieved with a high NH 3 yield rate of 13.94 ± 0.39 μg h -1 mg -1 at -0.15 V vs RHE and a high Faradaic efficiency of 12.72 ± 0.58% at -0.1 V vs RHE. Density functional theory (DFT) calculations reveal that the Mo 2 C (001) surface has a strong N 2 adsorption energy of -1.47 eV with the side-on configuration, and the N≡N bond length is elongated to 1.254 Å, indicating the enhanced N 2 adsorption and activation on the Mo 2 C surface. On the other hand, the low Δ G H* for HER over the MoO 2 (-111) surface demonstrates the impeded HER process for MoO 2 . This work may provide effective catalyst-design strategies for enhancing the electrocatalytic NRR performance of Mo-based materials.