Numerical analysis is conducted to clarify chemical effects of added steam to either fuel-or oxidizer-side on flame structure and NO emission behaviour with detailed chemistry in hydrogen-oxygen-nitrogen diffusion flames. An artificial species, which has the same thermodynamic, transport, and radiation properties to added H 2 O, is introduced to feasibly isolate chemical effects of added H 2 O. It is found that the reaction step (-R23) is the starting point to induce chemical effects of added steam. Special concern is, thus, focused on the impact of OH radical on flame structure and NO emission behaviour. A strong dependency of the amount of steam addition on OH radical behaviour is clearly displayed, and this modifies flame structure sufficiently to produce higher flame temperature at more than a certain mole fraction of added steam in comparison to that diluted with artificial species. It is also shown that the reaction step (-R23) is closely related to flame temperature and thereby the location of maximum flame temperature. The behaviour of NO emission index is shown to be greatly influenced by the competition between the reaction steps of (R63) and (R65) in addition to Zeldovich NO. It is, consequently, seen that the intermediate active species, HNO, affects NO emission behaviour remarkably. These results may be helpful to understand the role of recirculated steam in the combustion systems with flue gas recirculation to either fuel-or oxidizer-side.