Abstract: Thermoplastic polyimide (TPI) composites modified by 20wt% MoS2 were taken as research object and the wear loss was measured by improved MP-X2000 type tribo-tester. A new method, which could determine the run-in wear stage, was proposed on the basis of the wear loss variation. Accordingly, the model of wear process was set up. The effects of the couple roughness and loading on wear behavior during the run-in stage were also investigated and the wear mechanism was determined by SEM. The results show that the run-in stage was extended while increasing the couple roughness or loading. The wear rate increased with increasing the load, and was relative small when the surface roughness was 0.34 μm. Due to the different actions on material wear behavior, the third body formation and remove process were conducted by the couple roughness and loading respectively. The abrasion wear was the main factor during the low load and adhesion wear became more important while the load reached to 0.75 MPa.