In the steam-injected combustion field where the Prompt mechanism controls NOx formation, a decrease in flame temperature physically affects the NOx reduction. In addition, it is anticipated that the changes of concentrations of active radicals (CHI OH, H, etc.) due to steam addition bring about a chemical kinetic effect on the NO, reduction. In order to investigate the chemical reaction effect of steam addition on NOx reduction, we carried out numerical calculations of counterflow diffusion flame using detailed chemical kinetics, when the maximum flame temperature were adjusted to the same value by varying the initial temperatures. Under the same maximum flame temperature, the NO emission index decreases with increasing amount of steam addition, and the concentration and mass production rate of CH, HCN and N, etc., related to NOx formation also decrease when steam is added. Consequently, we clarify chemical kinetic effect of steam addition on NOx reduction.