Although abnormal wear in EGR-Diesel engines is well known, the primary reason has not been clarified. In the present study, concentration of burnt gas, oxidation of oil, and condensed water in the spaces of the piston ring belt were investigated through both theoretical calculation and chemical analysis of the sampled gas, oil and. water, using a pipe along the linkage to obtain the samples. It was demonstrated that the burnt gat concentration with EGR was significantly higher than that without EGR and that, as a result, oil TUN and condensed water pH value were drastically decreased with EGR.