Exhaust Gas Recirculation (EGR) system reduces exhaust emissions and improves fuel efficiency. Recently, the number of EGR system installed vehicles has been increasing, especially, for gasoline engine system. One of the major issues which decrease EGR function is a 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. However, there is less previous study on the EGR deposit with gasoline fuel but diesel fuel.In this study, influence of gasoline fuel compositions, especially, aromatics which was one of major component of EGR gas, and chemical structures on the deposit accumulation was investigated. To clarify the accumulation mechanism of EGR deposit, a thermal oxidative degradation test with an autoclave unit and an actual gasoline engine test were employed. Different composition fuels in control of 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. And 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 carbon number of the aromatic.