Single atom catalysts (SACs) with isolated metal atoms dispersed on supports exhibit distinctive performances for electrocatalysis reactions. The designable realization of well-dispersed single metal atoms is still a great challenge owing to their ease of aggregation. Here, Mo single atomic sites (Mo-N3C) combined with some ultrasmall Mo2C/MoN clusters (Mo-SA/Mo2C-MoN-Cs, mean diameter＜2 nm) on nitrogen-doped porous carbon were synthesized via a simple pyrolysis of bimetallic Zn/Mo metal-organic frameworks. X-ray absorption near edge spectra (XANES) in combination with various characterizations show that most of Mo species in sample exist in the form of single sites and the exact structure is Mo-N3C. Density functional theory (DFT) calculation further shows that as the number of N-coordination in the Mo-NxC moieties increases, the positive charge of Mo atoms increases. The single Mo atoms in Mo-N3C have the best capability of N2 adsorption, which may serve as main active sites for further electrochemical N2 reduction.