As a renewable energy source, ammonia is regarded as one of the ideal gases that can replace fossil fuels and has been extensively studied in large-scale combustion. However, studies on energy conversion efficiency and NOx emission in microscale are still inadequate. In this work, ammonia/oxygen premixed cylindrical micro-combustors with inner ribs under condition of lean combustion is numerically investigated. The key geometrical parameters of the ribs and the ammonia/oxygen equivalence ratio are evaluated based on thermal performance and NOx emission performance. Finally, the sensitivity analysis of NO and related reaction pathways are analyzed under different equivalence ratios. The results show that increasing the height of the rib and decreasing the distance between the first rib and the inlet can effectively inhibit the generation of NO. Among all cases, the combustor with U-shaped ribs is observed the minimum mole fraction of NO at the outlet under the same working condition, which is 16% less comparing to the rectangular-shaped one. Besides, the mean wall temperature shows a weak correlation with NO emission. Increasing the equivalence ratio can help gain higher mean wall temperature, but at the same time promotes NOx production. This study is helpful to the design and improvement of micro-combustors fuelled by ammonia.