Numerical simulation to grasp the flame structure and NO emissions in a H-2-air counterflow diffusion flame diluted with CO2 is conducted for a wide range of atmospheric air temperatures and highly preheated air temperatures. Special concern is given to the important role of the chemical effects of added CO2, especially in highly preheated air temperature flames diluted with CO2. There exists a limit of the oxidizer-side temperature below which flame cannot be sustained. It is observed in highly preheated air temperature flames that intensely diluted cases with CO2 show extremely low NO emission levels. The chemical effects of added CO2 reduce flame strength. It is also seen that the difference between the maximum flame temperature and the preheated air temperature becomes smaller and smaller with increasing preheated air temperature and mole fraction of added CO2, and this thus implies the acquisition of evenly distributed gas temperatures in industrial furnaces. The NO emission index increases as the oxidizer-side temperature increases and decreases as the mole fraction of added CO2 increases. The chemical effects of added CO2 suppress NO emissions, mainly because of the reduction of thermal NO. It is also stressed that the reaction N + CO2 - NO + CO, which is represented herein as reaction step (R283), is a relatively important contributor to prompt NO production.