Extreme pressure behaviors of a PN additive in mineral oil(MVI150),polyαolefin(PAO4),synthetic ester and polyether were studied by fourball friction and wear test machine.The effects of additive concentration on extreme pressure properties of base oil were also studied.The results show that the weld loads of MVI150,PAO4 and synthetic ester incorporated with 30% PN additive are more than 8 000 N,and the maximum nonseizure loads of MVI150 and PAO4 are increased to 1 140 N and 1 710 N respectively when the concentration of PN additive is 025%,which indicates that the PN additive can significantly enhance the load carrying capacity of the base oils.The chemical composition of the rubbing surface of steel ball was analyzed by Xray photoelectron spectroscopy,and the extreme pressure mechanism of the PN additive was also discussed.The result reveals that the excellent extreme pressure performance of the PN additive can be attributed to the formation of lubricating film with good loadcarrying capacity on the metal surface by tribochemical reactions between abundant P and N elements in the additive and metal surface during the rubbing process.