The effects of fuel properties and emission control systems on exhaust hydrocarbon emissions have been studied.Using fourteen fuels with different properties, exhaust hydrocarbon emissions were measured for the two vehicle types with different emission control systems, under body catalyst and closed coupled catalyst, under the Federal Test Procedure. The fuel properties included high and low concentrations of olefins and aromatics as well as high and low T90. In addition, two fuels contained MTBE.The hydrocarbon emissions were discussed from the view point of the ozone reactivity and ozone formation potential. The results show that the high ozone reactivity of exhaust emissions are mainly caused by the olefins and aromatics in fuels. And also, the effects of fuel property change on exhaust emissions for the vehicle with an under body catalyst are more sensitive than the case of the vehicle with a closed coupled catalyst.To attain the fuel properties with low ozone reactivity emissions, the effects of each fuel species on each exhaust hydrocarbon species were analyzed by a regression analysis method expressed by a matrix.From theses studies, It is concluded that benzene, Tri- alkylbenzene, C5-C6 olefins and C4-C6 paraffins in fuels are main source to produce the Di,Tri alkylbenzene and C2-C4 olefins which have the high ozone reactivities in the exhaust emission.