Ammonia (NH3) is receiving considerable attention as a potential substitute for carbon-containing fuels to reduce CO2 and soot emissions. Co-burning with hydrocarbons is a feasible solution to improve the combustion performance of NH3, which however, brings new challenges such as NO emissions. Hence, it is essential to investigate the detailed suppression/formation mechanisms of soot and NO during the co-burning of NH3 and hydrocarbons. In this study, the effects of NH3 addition in the fuel stream on soot and NO formations in a CH4/air co-flow atmospheric-pressure diffusion flame were numerically investigated using a 2D code and a combined chemical mechanism comprised of a model of polycyclic aromatic hydrocarbons (PAHs) up to five-rings and detailed nitrogen-containing reactions. The integral of reaction rates over the whole flame were obtained in both CH4 and CH4single bondNH3 co-flow flames to quantitatively investigate how the NH3 addition affected the soot and NO formation pathways. The results showed that the addition of 10% NH3 in the fuel stream of a CH4/air diffusion flame had a strong suppression on soot formation, decreasing the peak soot volume fractions by 38.9%. It was found that the NH3 addition led to decreases in CH4 and H mole fractions close to the burner exit, and hence lowered the integral of reaction rate of R67 (CH4+H=CH3+H2) by 1.07×10−5 mol/s, which in turn reduced the A1 production mainly through the pathway of CH4→CH3→C2H6→C2H5→C2H4→C2H3→C2H2→C3H3→A1. NO and NH played an important role in the consumption of C1∼C2 species involved in A1 formation pathways. The NH3 addition led to an increase in the integrated rates of CH3+NH=CH2NH+H and C2H+NO=HCN+CO by 2.45×10−6 and 2.59×10−6 mol/s, respectively. The decrease of A1 mole fraction reduced the inception, condensation and HACA surface growth rates. The peak NO mole fraction was increased by two orders of magnitude, from 28 ppm in the CH4 flame to 3060 ppm in the CH4single bondNH3 flame. A promoting effect of NH3 addition on N2O was observed in CH4single bondNH3 mixtures, however, through different channels than that of CH4 flames. In addition, more CN species, such as CH2NH, H2CH and CH2CN, were produced in the CH4single bondNH3 flame compared to the CH4 flame.