The influence of the ionic liquid used as the additive on the tribological properties of castor oil is investigated using a four-ball test rig. The additive percentages of ionic liquid in castor oil are 0.6 wt.%, 1 wt.%, 2 wt.%, and 3 wt.%. The test results show that a small amount of ionic liquid can postpone the peak coefficient of friction and decrease the wear behavior of castor oil. The minimum of the wear scar diameter (WSD) occurs at 1 wt.% ionic liquid. It seems that the chemical adsorption of castor oil and ionic liquid on the steel surface is more significant than the physical adsorption. This study should be of great importance of understanding the potential engineering application of castor oil.