Abstract: The content of unsaturated double bonds (C=C) of soybean oil (SO) was decreased by argon plasma polymerization to obtain polymerized oil PAR2. The molecular weight distribution of PAR2 was determined with a gel permeation chromatograph. The tribological properties of PAR2 and 175BS,a bright lubricating oil with the same viscosity grade as that of PAR2,were evaluated with a four-ball machine. Moreover,the chemical composition and state of the elements on the sliding surface of the steel balls were analyzed by means of X-ray photoelectron spectroscopy. Results indicate that Ar plasma treatment increased the viscosity of PAR2 at 40℃ from 33.8 cSt to 956 cSt. As compared with 175BS,PAR2 exhibited better viscosity-temperature and low temperature properties. The polymerized species in plasma-treated PAR2 mainly included the dimers and oligomers of SO,and the number average molecular weight,M_n,of the dimers and oligomers were 2 735 and 5 434,respectively. Moreover,PAR2 showed better load-carrying capacity as well as friction-reducing performance and anti-wear ability than 175BS. This is attributed to the synergistic effect of iron carboxylate and iron oxide with high valent state on sliding steel surface,thereby effectively reducd friction and wear of the steel-steel contact.