Aiming at the study of rare earth mischmetals as lubricant additives,an new oil-soluble lanthanum dialkldithiophosphate(LaDDP) is prepared as antiwear and extreme pressure additive,and its tribological performances are evaluated by four-ball mechine.The tribological decomposition reactions are simulated through mass spectroscopy.The tribochemical characteristics under boundary lubrication conditions are studied by means of AES and XPS.The tribological mechanism is suggested.The experimental results indicate that LaDDP possesses better lubrication ability than ZDDP.This can mainly be attributed to the formation of boundary film containing La,La 2O 3,FeS,sulphate,phosphate and lanthanum diffused layer.It indicates that the triblogical mechanism suggested is reasonable.