Abstract: In this study soot particles were collected from diesel engine exhaust pipe. Their morphology, structure as well as functional groups were investigated by means of scanning electron microscopy/energy-dispersive spectrometry, high resolution transmission electron microscopy, X-ray diffraction, Raman spectrometry, and X-ray photoelectron spectroscopy methods. A SRV IV oscillating reciprocating friction and wear tester was used to investigate the effect of diesel soot on the tribological behavior of CF-4 15W-40 oil. Results show that diesel soot consisted mainly of carbon and oxygen with an average size of about 34 nm, which shared the onion-like structure with several graphitic layers in the out shell. When the mass fraction of diesel soot in CF-4 15W-40 was below 3%, it improved the friction reduction but had little effect on the antiwear property. As the mass fraction of soot exceeded 3%, friction and wear increased. The possible tribological mechanism was that because of the special onion-like structures diesel soot rolled at the tribo-interface as a ball in a ball bearing, which decreased the friction coefficient. However, higher amounts of diesel soot in lubricating oil broke the lubrication film and blocked the lubricating oil entering into frictional interface, which resulted in high wear and increased friction coefficient.