The study investigated the formation characteristics of nitrogen oxides (NOx) in non-premixed coflow methane jet flames experimentally and numerically. Ammonia was added to the fuel stream, while varying the oxygen ratio in the oxygen/nitrogen oxidizer stream. In the coflow jet flame experiment using pure oxygen oxidizer, NOx emissions increased monotonically with respect to the mixing ratio of ammonia. When using oxygen/nitrogen oxidizers, NOx emissions reached a maximum at an oxygen ratio of 0.7, with non-monotonic changes occurring in response to variations in the oxygen ratio. Depending on the ammonia addition, increasing NOx emissions appeared in the two ranges of either relatively low or high ratio oxygen in the oxidizers. However, a reversal phenomenon of decreasing NOx emissions was noted within the section between the two ranges of the oxygen ratio. To elucidate the characteristics of NOx formation under various conditions of fuel and oxidizer compositions, 1-D and 2-D numerical simulations were conducted using a detailed chemical kinetics (GRI-Mech 3.0). The 2-D simulation results for pure oxygen and oxygen/nitrogen oxidizers provided reasonable predictions of trends in experimentally measured NOx emissions with the ammonia addition. Moreover, the cause of the reversal phenomenon was successfully explained through a comparative analysis of reactions leading to production and destruction of nitrogen monoxide (NO), based on the results of the 1-D and 2-D simulations. The results provide fundamental information to help in the design of oxygen-enriched combustor. (C) 2016 Elsevier Ltd. All rights reserved.