HRPB350 (highly reactive polyisobutylene, m.w.=350) derivatives were synthesized by the reaction of some compounds having S and N atoms and HRPB350. These are widely used as lubricant additives and fuel detergents. In order to study the friction reducing effect of these derivatives, we measured the size of the 4-ball WSD(wear scar diameter). The 4-ball WSD was significantly reduced when the HRPB350 derivatives was added in base oil compared to the case without additives. It was also confirmed that friction reducing effect of these derivatives was similar to that of the industrial products. Micrographs of worn scar when lubricating (a) without, and (b) with 1% of an HRPB350 derivative in base oil at the load of 392 N and 1 hour