Abstract: Two ammonium phosphate ionic liquids, N.N-Tridodecyl-N-hexadecyldi(2-ethylhexyl) ammonium phosphate (N22216P) and N.N-triethyl-N-hexadecyl di(2-ethylhexyl) ammonium phosphate (N12121216P) were prepared. The dissolution performance, the viscosity temperature performance, thermal stability performance and corrosion performance of two ionic liquids were evaluated in PAO10. The results indicated that the N22216P and N12121216P could be dissolved in base oil PAO10. And the viscosity temperature performance of both ionic liquids was better than that of PAO10. The thermal stability performance of 1% N22216P and 1%N12121216P was higher than that of PAO10 and 1% T204. These results showed that the viscosity temperature performance and thermal stability of ionic liquids were enhanced with the increase of alkyl chains. The addition of ionic liquids additives could reduce the corrosion of base oil on friction pairs. What’s more, the tribological properties of the N22216P and N12121216P as the additives of PAO10 were explored at room and high temperatures. The effect of alkyl chain structure of the prepared ionic liquids was studied on their lubrication performance. Compared with the commercial additive 1% T204, it was found that 1% N22216P and 1% N12121216P exhibited a better lubrication performance and abrasion resistance at room temperature and high temperature. The electrical contact resistance (ECR) measurement, energy dispersive spectrometer (EDS) measurement, Quartz Crystal Microbalance (QCM) measurement and X-ray photoelectron spectroscopy (XPS) measurement were carried out to understand the lubrication mechanism of two ionic liquids as additives more clearly. Based on these characterization results, it was speculated that these two ionic liquids were prone to adsorb on the substrate surface to form an adsorption film during the friction process. This helped to form a dense friction protective film on the steel surface through further frictional chemical reactions. This protective film effectively prevented the direct contact of sliding pairs and played an important role in reducing friction and anti-wear. In addition, the N22216P with lower steric hindrance structures were easier to adsorb on the substrate to form a denser adsorption film according to the results of the QCM. And further formed more dense lubrication films containing anti-wear compounds on the steel surface by the reactions. Thus, the N22216P with smaller steric hindrance had better lubrication performance than that of the N121216P. This indicated that the lubrication performance of ionic liquids as an additive was influenced by the steric hindrance structure. Therefore, so-prepared two ionic liquids (N22216P and N12121216P) with efficient tribological properties would improve the comprehensive performance of base oil to produce a cost-effective lubricant additive applied in oil-based fluid.