Abstract: In order to improve the lubricating properties of vegetable oil, the probability was discussed that ionic liquids used as additives in vegetable oil. The base oil and additive were castor oil and 1-hexyl-3-methylimidazolium tetrafluoroborate ionic liquids (HMIMBF₄), respectively. Lubrication properties of mixed castor oils were investigated using the ultraviolet visible spectrometer, rheometer, thermo gravimetric analyzer, four-ball friction testing fig, SEM and EDS, respectively.The mechanism was explored that theHMIMBF₄ played an role on the castor oil.The results showed that the impact on the viscosity-temperature property of castor oil by theHMIMBF₄ grew smaller gradually with elevated temperature, and the HMIMBF₄ also increased the thermal stability of the castor oil.Small amount of HMIMBF₄ improved the largest seizured load, sintered point load and contact pressure of castor oil, but a more number of HMIMBF₄ easily deposited on the ball surface and reduced the wear resistance and load-carried capacity of the castor oil.