Abstract: Micro sample four-ball friction and wear tests were conducted to evaluate the action mechanism of olefin sulfide (SO) as an additive in liquid paraffin. The residue lubricating oil was analyzed qualitatively by means of gel permeation chromatography (GPC) and inductively coupled plasma (ICP) atomic emission spectrometry. An X-ray photoelectron spectrometer (XPS) was used to investigate the tribochemical reaction of SO with the steel pair. As the results, no friction polymer is detected by GPC in the residue lubricating oil at a relatively low load of 98N, and the friction-reducing and antiwear capacity in this case is mainly attributed to the chemisorbing of SO onto the metal surface. Under medium load of 196N to 294N, the formation of friction polymer of high molecular weight is confirmed by GPC spectra of the residue lubricating oil, and the friction-reducing and antiwear capacity in this case is attributed to the friction-induced polymer. Under a relatively high load of 392N, the friction polymer experiences degradation and less Fe is detected in the residue lubricating oil, while the inorganic boundary film composed of FeS and FeSO 4 contributes to improve the friction-reducing and antiwear behavior in this case.