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Electrocatalytic urea synthesis from CO 2 and NO 3 − are environmental green and energy savable. Although Cu are effective for CO 2 and NO 3 − reduction reactions, achieving high efficiency of urea production remains challenging. Herein, Cu-Ni nanocrystals are formed to provide heterogenous active sites for promoting hydrogenation and C−N coupling reactions in urea synthesis. The carbon supported Cu-Ni nanocrystals are characterized by extended X-ray absorption fine structure and X-ray photoelectron spectroscopy spectra. The Cu-Ni nanocrystals achieves the highest urea Faradaic efficiency of 25.1 % and urea yield of 22.01 μmol h−1 cm−1, which is much higher than that of single Cu or Ni metal. The reaction mechanism is probed by in-situ Fourier transform infrared spectroscopy, revealing a C−N coupling reaction pathway between *CO and *NH 2. Theoretical calculations suggest that hydrogenation of *NO x and coupling *CO with *NH 2 are more easily occurred over co-existed Cu and Ni sites rather than homogeneous Cu or Ni site. [Display omitted] ● The Cu-Ni alloy show much enhanced electrocatalytic activity than metals of Cu and Ni in urea synthesis from NO 3 - and CO 2. ● Mechanism of urea formation is investigated by FT-IR, showing a possible C-N coupling reaction pathway between *NH 2 and *CO. ● The heterogeneous Cu/Ni atoms are more reactive in *NO x hydrogenation and *CO/*NH 2 couplings than Cu or Ni atoms. [ABSTRACT FROM AUTHOR]