Ammonia (NH₃) is considered a promising zero-carbon fuel and was extensively studied recently. Mixing high-reactivity oxygenated fuels such as dimethyl ether (DME) or dimethoxymethane (DMM) with ammonia is a realistic approach to overcome the low reactivity of NH₃. To study the combustion characteristics of NH₃/DMM and NH₃/DME mixtures, we constructed a NH₃/DMM chemical mechanism and tested its accuracy using measured laminar burning velocity (LBV) and ignition delay time (IDT) of both NH₃/DMM and NH₃/DME mixtures from the literature. The kinetic analysis of NH₃/DMM flames using this mechanism reveals that the CH₃ radicals generated from the oxidation of DMM substantially affects the oxidation pathway of NH₃ at an early stage of flame propagation. We investigated the formation of nitrogen oxides (NO_x) in NH₃/DMM and NH₃/DME flames and little difference can be found in the NO_x emissions. Using NH₃/DMM flames as an example, the peak NO_x emissions are located at an equivalence ratio ($\phi$) of 0.9 and a DMM fraction of 40% in the conditions studied. Kinetic analysis shows that NO_x emission is dominated by NO, which primarily comes from fuel nitrogen of NH₃. The addition of DMM at 40% significantly promotes the reactive radical pool (e.g., H, O, and OH) while the maintaining a high concentration of NO precursors (e.g., HNO, NO_2, and N₂O), which results in a high reaction rate of NO formation reaction and subsequently generates the highest NO emissions.