This paper presents the results of fundamental research on gas turbine cogeneration systems with steam injection that attains high thermal efficiency and low pollutant emissions. The concentration of OH radical may be affected significantly in these systems. To prove the behavior of OH radical in the combustion zone and to clarify the relationship between OH radical production and NOx emission, the characteristics of methane-air counterflow flames with steam addition have been simulated numerically by applying GRI-Mech detailed chemical kinetics. To investigate the chemical reaction effects of steam addition on the OH radical and NO formation with the effect of the temperature decrease because of steam addition excluded, the maximum flame temperature was adjusted to the same value by varying the initial temperature. The calculated results clearly show that with steam addition, the decomposition reaction of steam is suppressed and the production rate of the OH radical is decreased regardless of the increase in steam concentration. These results can be explained by the significant decrease in flame temperature for the same initial temperatures of fuel and oxidizer. For the same flame temperature, the OH concentration becomes higher. The increase in the OH concentration because of steam addition in itself results in the increase in NOx emissions. However, because of the decrease in CH concentration with steam addition, the production rates of HCN and N radicals decrease dramatically, and NO formation is suppressed as a whole. These results may be useful to predict NOx emissions in a high-temperature gas turbine system with steam or water addition.