combustor fueled with premixed ammonia/oxygen using three-dimensional numerical simulations. For this, a three-dimensional model is developed with a detailed chemical kinetic mechanism. The effects of 1) the presence/ absence of the bluff-body, 2) its dimensionless height h and 3) its dimensionless axial location l are examined. It is found that the implementation of a bluff-body can significantly affects the thermal and NO emission characteristics. Up to 39.1 % reduction of NO formation and 41.6 K increase in the outer wall temperature (OWT) can be achieved as the bluff-body is optimized, when compared to the case in the absence of the bluff-body. Furthermore, increasing h leads to OWT being increased, but it can suppress the NO generation in some cases. In addition, increasing l is shown to be involved with a high OWT and a low NO concentration, mainly due to the variation in the flow field and thus heat transfer characteristics. Finally, the dimensionless pressure loss is strongly affected by the NH3 flow rate, and the dimensionless parameters h and l. This work reveals that implementing a proper designed bluff-body is an effective way to enhance thermal performance and reduce NOx emission.