Exhaust Gas Recirculation (EGR) systems reduce exhaust emissions and improve fuel efficiency. Recently, the number of EGR system installed vehicles has been increasing, especially for gasoline engine systems. One of the major causes of decreasing EGR function is deposit accumulation on a gas passage. The deposit consists mainly of hydrocarbons which are degradation products of fuel, thus the amount of deposit seems to be strongly affected by fuel compositions. Unfortunately there are not as many studies on EGR deposits with gasoline fuel as there are with diesel fuel. In this study, the influence of gasoline fuel compositions, especially aromatics which are major components of EGR gas, on chemical structures of the deposit were investigated. To clarify the accumulation mechanism of EGR deposits, a thermal oxidative degradation test with an autoclave unit and an actual gasoline engine test were employed. Different composition fuels to control the carbon number of the aromatic were prepared as test fuels. The result of the thermal oxidative degradation test showed that the higher carbon number of the aromatics in a fuel increases the accumulation of deposit. The result of the engine test indicated that the test fuel, containing higher amount of C10 and C10+ aromatics, caused higher accumulation rate of deposit. These results suggested that the accumulation of EGR deposit was affected by the carbon number of the aromatic.