Abstract: In this study, two 2-mercaptobenzothiazolate based phosphonium ionic liquids were synthesized. The antioxidation and tribological properties of these ionic liquids in dioctyl sebacate (DOS) base oil were investigated by comparison with traditional additive ZDDP. Thermogravimetric analysis (TGA) test was conducted to investigate the thermal stabilities of ionic liquids. The test results showed that the thermal decomposition temperatures of P8 and P10 ionic liquids were higher than that of DOS ester oil, indicating that both ionic liquids had high thermal stability. Rotating bomb oxidation tests were utilized to investigate the antioxidation properties of two ionic liquids in DOS base oil according to the standard methods. The results showed that two ionic liquids possessed high antioxidation performances. They could prolong the oxidation induction time of base oil significantly even at a low concentration of 0.5 %. And the enhancement effects were better than that the traditional additive ZDDP. When the concentrations of the two ionic liquids increased from 0.5% to 1%, the oxidation stabilities of oil samples were significantly increased. In addition, the oxidation induction time of P8 ionic liquid was longer than that of P10 ionic liquid, indicating that P8 ionic liquid had better antioxidation performance. The excellent antioxidation performances of two ionic liquids were attributed to the chelation of 2-mercaptobenzothiazolate anion with copper ions. The formation of chelate reduced the catalytic oxidation rate, which further improved the oxidation stability of oil sample. The extreme pressure properties of two ionic liquids in DOS base oil were investigated by standard methods. The P_B values of oil samples with different additives were tested. The results indicated that two ionic liquids showed better extreme pressure performances than ZDDP at both concentrations of 0.5% and 1%. P8 ionic liquid obtained the highest values of P_B at the concentrations of 1%, indicating the highest extreme pressure performance. Tribological experiments were carried out on SRV-V to study friction-reducing and antiwear properties of two ionic liquids in DOS base oil. The results indicated that the friction coefficient curves of P8 and P10 ionic liquids at two concentrations were similar. The length of cationic alkyl chain had little effect on the friction-reducing properties. While the P8 ionic liquid possessed lower wear volume, indicating better antiwear property. What’s more, the friction-reducing and antiwear properties of P8 ionic liquid were better than those of the traditional additive ZDDP. To investigate the mechanisms of ionic liquids with excellent tribological performances, the worn surface morphologies were researched by SEM and the elements distribution and contents were detected by EDS. The results of SEM and EDS indicated that the tribochemical reactions were occurred between ionic liquids and frictional pairs during the friction process. The tribochemical products containing S element were produced. Both the tribochemical products and the oxides on the worn surface formed boundary lubricating film, which resulted in the excellent friction-reducing and antiwear performances in DOS base oil. Therefore, the two 2-mercaptobenzothiazolate based phosphonium ionic liquids possessed excellent antioxidation, friction-reducing and antiwear properties in ester base oil. These ionic liquids as multifunctional additives showed great potentials to be used in ester oil in the future.