Abstract: The antiwear properties of sodium metaborate as an oil additive? the effects of dispersant and hydration water of sodium metaborate on its performance have been studied using a Four-ball Machine and a Timken Tester. XPS and AES were used to study the rubbed surfaces. It is shown that sodium metaborate possesses a good antiwear function due to the formation of a film containing boron, oxygen, sodium, carbon and iron. The existing form of boron on the rubbing surfaces is sodium metaborate which can be dissolved in water. The hydration water of so- dium metaborate can reduce the aiitiwear function on Four-ball Machine but enhance its antiwear ability on Timken Tester. The study also revealed that sulfonate dispersant Can reduce the antiwear function of sodium metaborate while succinimide dispersant enhance its antiwear ability.