Tribological properties of tricresyl phosphate(T306)and ammonium thiophsphonate(T307)as extreme pressure and antiwear additives in diester base oils were performed by using a fourball tester.The morphologies of the worn surface and the chemical state of elements on the worn surface were investigated by scanning electron microscopy(SEM)and Xray photoelectron spectroscopy(XPS).The results show that two additives can improve tribological properties of diester base oils in different degrees under the investigated test conditions such as additive content and test load.In particular,T307 exhibits superior friction reducing,antiwear and extreme pressure properties in diester base oils than T306.SEM and XPS results show that excellent tribological performance can be attributed to the formation of boundary lubrication films containing elements S,P,N on the rubbing surfaces. 