Air dilution in highly preheated air combustion has been found to be effective in controlling the NOx emission and improving the temperature uniformity. In this study, the effects of air dilution with different diluents on highly preheated air combustion have been numerically investigated. A regenerative furnace was simulated, where nitrogen, carbon dioxide, helium, argon, and flue gas were adopted as air diluents. It was found that the flame temperature, the temperature uniformity, and the NOx emission are different when different air diluents were adopted under the same preheated air temperature and oxygen concentration. On the basis of the analysis and comparison, it is deduced that the heat capacity of the diluted air, represented by the product of the density and specific heat, plays a main role in the effect of air dilution on the combustion process. The turbulent intensity, represented by the kinetic energy, also plays a role but less significant compared with the heat capacity. The difference in the momentum of diluted air flows with different diluents results in a change in the flame shape.