Recently, NO formation and its reduction technology with steam addition in fuel-rich combustion have been studied in detail. However, only a few studies have been made of the NO formation mechanism in a combustion field of hightemperature and highequivalence ratio with steam addition such as steam injection-type gas turbine. In this study, numerical analysis of a fuel-rich methane/air counterflow premixed flame was carried out using detailed chemical kinetics. First, combustion characteristics without steam injection equivalence ratios 1.3 and 2.0 were simulated, and the effect of equivalence ratio on NO formation mechanism was discussed. Then, the reduction mechanism of NO formation in fuel-rich combustion caused by steam addition was analyzed. The results showed that the steam addition decreased the flame temperature and reduced NO formation, and that these effects decreased with the increase of equivalence ratio. These effects of steam addition on NO formation were found to be caused by two factors: the physical effect of decreasing the flame temperature, and the chemical kinetic effect of decreasing the concentration of CH radicals due to the reaction with added steam.