This paper presents the effects of swirl ratio on the reactivity controlled compression ignition engine parameters with a focus on NOx reduction. A numerical effort with a Eulerian-Lagrangian methodology is implemented to simulate the engine cycle. The accurate prediction of NOx is essential. Therefore, four different NOx mechanisms are compared and the 29 step-reaction mechanism is chosen for the current investigations due to its minimum error. It is found that the increase of swirl ratio expedites the combustion process. Therefore, the heat rejection to the cooling water during the combustion is decreased. The heat transfer after the end of combustion is also increased due to the enhancement of heat transfer coefficient. A tradeoff between these two opposite effects is achieved around the swirl ratio of 0.95-1.2 that the best efficiency and lowest NOx emission is achieved. It is also notable that at the minimum combustion duration, the average bulk temperature of engine cycle is minimized. (C) 2019 Elsevier Ltd. All rights reserved.