The effect of water addition on NO formation in counterflow CH4 /air premixed flames was investigated by numerical simulation. Detailed chemistry and complex thermal and transport properties were employed. The results show that the addition of water to a flame suppresses the formation of NO primarily due to flame temperature drop. Among a lean, a stoichiometric, and a rich premixed flame, the effectiveness of water addition is most significant for the stoichiometric flame and least for the rich flame. The addition of water also reduces the formation of NO in a flame because of the chemical effect. Compared to the stoichiometric flame, the chemical effect is intensified in the lean and rich flames.