Unlike hydrocarbon fuel, ammonia is a carbon-free and renewable energy source. It is also regarded as one of the potential energy carriers. However, ammonia combustion for power generation is not well studied under micro-scale conditions, especially concerning nitrogen oxides (NOx) emission. For this, thermal performances and NO emission characteristics of premixed ammonia/oxygen combustion are numerically investigated on a micro-planar combustor. The effects of 1) the equivalence ratio ϕ, 2) inlet temperature Tin and 3) inlet pressure Pin are examined. The outer wall mean temperature (OWMT) is found to vary non-monotonically as the mixture varies from lean to rich conditions, with the peak occurring at ϕ = 0.9 mainly due to the optimal heat transfer performance. However, a low ϕ could lead to the high nitric oxides (NO) concentrations because of the high flame temperature as well as O atom concentrations. Up to 75.5% of NO reduction could be achieved, as ϕ is optimized. Furthermore, increasing Tin is shown to be associated with a low OWMT and NO concentration. In addition, varying Pin is shown to lead to not only OWMT being changed but also NO formation being mitigated. The decrease in NO concentration for high Pin is mostly attributable to the short residence time of high flame temperature in the channel and low OH concentrations. This work reveals that optimizing the operating thermodynamic parameters is an effective means to reduce NO emissions and improve thermal performances.