Diesel engines equipped with exhaust gas recirculation reduce NOX emissions; however, engine combustion causes large amounts of soot to be produced. This soot intermixes with and contaminates lubricants thereby adversely affecting engine durability.We focused our attention on the behavior of soot with EGR and the process by which soot intermixes with lubricant. Our study indicated that the amount of soot intermixing with lubricants is related to exhaust smoke and the EGR rate. Most of the soot intermixes with the oil film on the cylinder wall. Intermixing with blow-by gas is very small.