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 the 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, since the dominant NO formation mechanism varies. The addition of water also reduces the formation of NO in a flame because of chemical effect that increases the concentration of OH, while reduces the concentrations of O and H. Compared to the stoichiometric flame, the chemical effect is intensified in the lean and rich flames. ; peer reviewed: yes ; NRC pub: yes ; admin: American Society of Mechanical Engineers (ASME) does not permit archiving of pre- and post-print. The Publisher's version/PDF cannot be used in the repository. Users must link to publisher' site. NRC could store the publisher's version as an archival copy