This work investigates the formation mechanism of the third bodies for brass under lubricated conditions. Atribometer based on on-line and radio-nuclide technique (RNT) techniques is used to perform the sliding ex-periment of brass versus steel. The microstructure, micro-hardness and chemical composition of the third bodiesare analyzed by mean of the scanning electron microscope (SEM), focused ion beam microscope (FIB), trans-mission electron microscope (TEM), hardness testers and X-ray photoelectron spectroscopy (XPS). The resultsindicate that the contact pairs show good tribological behaviors and the layered third bodies (mechanicallymixed layer (MML), grain refinement zone (GRZ) and plastic deformation zone (PDZ)) are formed on the brasssurface during sliding. The formation mechanism of the third bodies is explored. The MML gradually forms onthe worn surface in the later stage of the test by proceeding of oxidation reactions. Due to the severe plasticdeformation caused by sliding, the interactional dislocation movements and dynamic recrystallization (DRX) lead to the formation of GRZ with nano-scale grains. The PDZ grains lead to the shear deformation by means offriction stress. ... mehrMoreover, the generation and accumulation of dislocations result in voids in the PDZ.