This work presents a theoretical study of the influence of mixing on NO removal from flue gas by the selective noncatalytic reduction (SNCR) process with CH4 or H-2 addition, using an elementary reaction mechanism together with a simple approach for mixing proposed previously. The results show that the mixing process affects NO reduction significantly. For he SNCR process with CH4 addition, the mixing process narrows the temperature window at the high temperature side. For the case with H-2 addition, besides the temperature window getting narrower, the maximum NO removal efficiency declines notably, and NH3-slip near the optimal temperature rises. This discovery explains the experimental results in the literature reasonably, and it indicates that fast mixing is essential to gain good performance, while H-2 additive is used in practical application. Furthermore, the reason for the distinct effects of mixing on the two DeNO(x) processes was revealed by theoretical analysis.