Different surface roughness by materials of intake pipe in a EURO 6 type diesel engine affected on internal flows and it’s effects were analyzed with VECTIS program. Basically, the internal flow could be improved by the smaller head loss of the plastic intake pipe with the lower surface roughness compared with the steel. In additions, at the outlet of the plastic intake pipe, the homogeneity of the velocity distribution was improved and larger turbulences were generated in the section than the steel. Therefore, it was confirmed that the flow characteristics with lower surface roughness of the plastic pipe would more improve the engine performance than the steel with larger surface roughness.