Abstract: Comparative running in bench test of EQ6100 engine was carried out to investigate the running in effect of conventional 15W/40 internal combustion engine oil and a specially formulated running in oil for the engine (coded as MR oil). A direct reading ferrograph TPD 2 was used to analyze the wear debris compositions of the tested oil sample after running in. The morphologies and elemental compositions of the surface of cylinder piston ring were observed and examined with a Talysurf 6 profilometer and a scanning electron microscope (SEM) equipped with energy dispersive spectrometer (EDAX). As the results, ultra dispersed diamond (UDD) in MR oil helps to accelerate the running in process and to smash larger wear debris in the running in oil to small ones, therefore to prevent the cylinder bore from abrasion by large hard wear debris. This contributes to improve the tribological properties of cylinder piston ring pair and the running in quality.