Flameless combustion or invisible flame in regenerative furnaces using highly preheated air has recently received much attention for its accomplishment not only in energy saving, but also for low nitric oxide emission. The characteristics of combustion with highly preheated air were studied to understand the change of combustion regime and the reason for the compatibility between high performance and low nitric oxide emission. It was found that combustion was sustained even in an extremely low concentration of oxygen, if the combustion air were preheated higher than the auto-ignition temperature of the fuel. As an application of the principle, we can reduce nitric oxide emission by dilution of the combustion air with plenty of recirculated burned gas in the furnace. Dilution makes the oxygen content of the oxidizer low, which decreases temperature fluctuations in the flame as well as the mean temperature, hence, low nitric oxide emission. Finally, the applicability of highly preheated air combustion to other fields than industrial furnaces has been discussed. (C) 2000 Elsevier Science Ltd. All rights reserved.