Flow reactor experiment has been performed to study the promotion effect of fuel species on the NO-NO2 conversion at reaction temperatures from 600 K to 1100 K. As the fuel species, n-butane (n-C4H10) and n-pentane (n-C5H12) were selected. The concentrations of NO, NO2, and fuel species in the flow reactor were measured using a reacting flow of air/NO (20 ppm) /fuel species (10 ppm) mixture at residence times up to 1.5 s. The dependence of the NO2/NOX ratios on the type of fuel species, the reaction temperature and residence time are discussed. The experimental results show that both butane and pentane strongly promote the NO-NO2 conversion even at the low reaction temperature (650 K). The most interesting results is the finding of the two peaks of the NO2/NOX ratios at lower and higher tempeartures. The appearance of the two peaks is thought to be due to the negative temperature coefficient of low temperature oxidation reaction for higher hydrocarbons.