To study the effect of magnetic field on the tribological properties of lubricating oils containing zinc butyloctyldithiophosphate(T202),an improved fourball tester was used to evaluate the tribological properties of T202 in the base oil of 150SN under magnetic and nonmagnetic field.The morphologies and the chemical states of several typical elements on the worn surfaces were examined by scanning electron microscope(SEM) and Xray photoelectron spectroscopy(XPS),and the preliminary tribological mechanism was discussed.The friction and wear test results indicate that the wear scar diameter and friction coefficient under magnetic field is all smaller than those in nonmagnetic condition.The XPS results indicate that complicated friction chemistry reaction occurs on the surface of worn surface lubricated with the lubricants containing T202,and magnetic field has positive effects on the combination of S,O,P and Zn elements with metal surface,which contributes to the forming of tribochemical film and the modification of worn surface,thus enhancing the lubrication effect.