The characteristics of NO_x formation and reduction were investigated in O_2/CO_2 combustion. As a result, CO and OH concentration in O_2/CO_2 combustion were drastically higher than air combustion. This was because high CO_2 concentration enhanced the reaction(CO_2+H→CO+OH). NO_x reduction mechanism arising from high CO and OH radical concentration condition was clarified. It was indicated that O_2/CO_2 combustion was suited for reducing nitrogen-oxide.