In order to study the friction degradation of lubricating oil and its influence on the performance of lubricating oil,four ball test machine was used to evaluate the tribological properties of triisooctyl trimellitate (TMTO),triisonony trimellitate (TMTD),triisotridecyl trimellitate (TMTD) under boundary lubrication condition.Results show that the tribological performances of the tested oils are highly influenced by the molecular structure.The friction and wear resistance of TMTT with the longest chain length is better than that of TMTO and TMTD.TMTD,which has the longest alkylchain,posses the best friction reducing and antiwear properties compared with TMTO and TMTD.Pressure differential scanning calorimetry (PDSC) and thermogravimetric analyzer (TGA) were applied to investigate the thermal and antioxidation behaviors of the lubricants,and the changes of the viscosities and acid values of the lubricants before and after tribological test were also investigated.Results indicate that the tribological degradation of trimellitate esters has great influence on the viscosity,acid value as well as antioxidation properties of the lubricants.The esters with good lubricating performances can prevent or slow down the oxidation process.Under the boundary lubrication condition,some of the trimellitate ester decomposes to the small molecule compounds due to mechanical shearing and hydrolysis,and some of the trimellitate ester is prone to polymerize and generate the high molecular compounds due to the oxidation.