Abstract: The effect of displacement amplitude on fretting wear behavior of copper-magnesium alloy was investigated. Fretting tests of Cu-Mg alloy were carried out on a multi-function fretting wear rig under different displacement amplitudes. The friction coefficient curve and F_t-D-N curve were recorded during the fretting process, and the damage area was analyzed by optical microscope, scanning electron microscope, energy dispersive X-ray spectrometer and three-dimensional profiler. The results showed that with the increase of displacement amplitude, the fretting contact condition changed from the partial slip regime to the gross slip regime, and the mixed slip state was not found. There was a transformation from the elastic deformation coordination to the plastic deformation coordination in the partial slip regime. The wear volume increased with the increase of displacement amplitude, in addition, the volume loss was very serious in the gross slip regime. In the partial slip state coordinated by elastic deformation, the damage of the contact surface was mild; while in the partial slip state coordinated by plastic deformation, high shear stress was produced in the contact area, and fatigue cracks extended to the contact surface resulting in peeling of material, with traces of abrasive and oxidative wear at the contact edge. In the gross slip regime, the contact surface damage mainly were fatigue delamination, abrasive wear and oxidation wear.