Abstract: Lubricating thin film of water soluble zinc alkyl thiophosphate formed in worn surface of four ball test has been studied by means of scanning electronic microsope, electronic probe analysis and scanning tunneling microscope. The results indicate that the film is non crystalline with high but almost the same content of elements P and Zn in some areas but dis well distributed S in the while wear scar. SEM and TEM analyses show that the lubricating film is a multi layer film with the thickness of each layer of about 500 nm, while the average dimension of the lubricating film is 15 000 nm or so.