During the mixing of hot combustion gas containing NO with cold air, NO2 is formed through the oxidation of NO. In this study, using hot combustion gas-cold air double concentric jets, the effect of methane and propane on this type of NO2 formation has been experimentally investigated. The methane (below 2530ppm) and propane (below 148ppm) were added to the cold air, and NO and NO2 concentrations were determined by a chemiluminescent analysis. The equivalence ratio of the combustion gas was varied from 0.76 to 1.20. It was found that the addition of low levels of methane and propane can significantly promote NO2 formation. Propane was found to promote NO2 formation much more effectively than methane. The difference of their effectiveness is discussed in view of hydrocarbon reactions in smog formation. From the above results, the level of hydrocarbons in the mixing region of combustion gas with air is one of the important factors controlling the direct emission of NO2 from combustion systems.