The effect of hydrogen addition to ultra lean counterflow CH4/air premixed flames on the extinction limits and the characteristics of NOx emission was investigated by numerical simulation. Detailed chemistry and complex thermal and transport properties were employed. The results show that the addition of hydrogen can significantly enlarge the flammable region and extend the flammability limit to lower equivalence ratios. If the equivalence ratio is kept constant, the addition of hydrogen increases the emission of NO in a flame due to the enhancement in the rate of the NNH or N2O intermediate NO formation routes. The addition of hydrogen causes a monotomic decrease in the formation of NO2 and N2O, except flames near the extinction limits, where the emission of NO2 and N2O first increases, and then decreases with the increase in the fraction of hydrogen. Overall, hydrogen enrichment technology allows stable combustion under ultra lean conditions, resulting in significant CO2 and NO emission reduction. ; peer reviewed: ; NRC Pub: Yes ; system details: machine converted author identifier PE to PID, February 2012