Numerical study is conducted to clarify flame structures and NO emissions in a single-staged (ammonia + methane)/air premixed combustion, which can be presumed with a counterflow configuration. To clarify the important role of downstream interaction on flame characteristics and NO emissions, injecting (80% methane+20% ammonia)/air mixtures to the ambiences of nitrogen and air is studied. When lean blended fuel/air premixed mixtures are injected to the ambiences of nitrogen and air, there are no significant change in flame structure and NO emission. When the rich premixed mixtures are injected to air (nitrogen) ambience, an interacting flame consisting of a rich premixed flame and a diffusion flame (a single rich premixed flame) is generated. The difference in such flame configurations completely alters flame structure and NO emission behavior. The present results can be utilized as a baseline case in reducing NO emission via various combinations such as ammonia-air, methane-air, pure air, and (ammonia+methane)/air in the secondary zone of a two-stage ammonia blended methane /air premixed combustor.